Gemara

(Beraisa - R. Yishmael) Question: "So you will bless Bnei Yisrael" teaches that the Kohanim bless Yisrael. What is the source that the Kohanim are blessed?

Answer (R. Yishmael): "I (Hash-m) will bless them."

R. Akiva: The verse teaches that Hash-m blesses Yisrael.

Megilah 23b (Mishnah): The following require at least 10 men... Chazaras ha'Shatz, Birkas Kohanim...

Sotah 38a (Beraisa) Question: "So you will bless Bnei Yisrael" - what is the source that they bless also converts, women and freed slaves?

Answer: "Say to them" - to all of them.

38b (Ada): If only Kohanim are in the Beis ha'Keneses, they all bless Birkas Kohanim.

(R. Zeira): They bless Yisre'elim in the fields.

Question #1 (Beraisa): People in back of the Kohanim are not blessed.

Answer: Those who could have come to face the Kohanim are not blessed. Those who were unable to are blessed.

Question #2: Rav Simi taught that if only Kohanim are in the Beis ha'Keneses, some bless and the others answer Amen!

Answer: If we can leave 10 to answer, then others give the Berachah. If there are only 10 in all, all bless Birkas Kohanim.

Kesuvos 8b (R. Yochanan): We bless Birkas Chasanim with 10, and Chasanim count towards the 10. We bless Birkas Avelim with 10, and mourners do not count towards the 10.

Rishonim

Rambam (Hilchos Tefilah 15:9): Birkas Kohanim requires 10. Kohanim are part of the Minyan. If only Kohanim are in the Beis ha'Keneses, all of them bless. They bless their brothers in the north and south. The women and children answer Amen. If 10 Kohanim remain apart from those who ascend (to bless), the 10 answer Amen and the others bless.

R. Mano'ach: The Yerushalmi says that if there were 10 at the beginning and some left, we finish. This applies to Chazaras ha'Shatz and everything that requires 10. It says about those who leave "v'Ozvei Hash-m Yichlu." Kohanim are included in the Minyan, for also they are blessed, just like we say that Chasanim are in the Minyan for Birkas Chasanim, for they are included in the Simchah.

Rashi (Sotah 38b DH Ishtayer) If 10 remain to answer, some Kohanim ascend to bless, and the others remain to answer. If 10 will not remain, they are not important enough to receive the Berachah themselves. Therefore, everyone ascends, and they bless Yisre'elim in the fields.

Kesef Mishneh: Based on this, we must say that the Rambam's law for when 10 remain applies also if there are more than 10 Kohanim. Ten do not ascend, and the rest ascend. The 10 answer Amen. Even though Birkas Kohanim is with 10, including the Kohanim, there is different, for Yisrael answer Amen. Here, if all are Kohanim, it is not considered Birkas Kohanim to answer Amen unless 10 answer. The Rambam says that when all the Kohanim bless, they bless their brothers in the north and south, and the women and children answer Amen. This is from the Yerushalmi.

Ran (Megilah 13b DH v'Ein Nos'in): It says here "Ko Sevarchu Es Bnei Yisrael", and Bnei Yisrael connotes 10 regarding a Davar shebi'Kedushah. We learn from "v'Nikdashti b'Soch Bnei Yisrael." This is only an Asmachta mid'Rabanan. The order of Tefilah itself is only mid'Rabanan!

Sefer Charedim (4:18, brought in R. Akiva Eiger): Yisre'elim who stand facing the Kohanim silently and intend to receive their Berachah, like Hash-m commanded, are part of the Mitzvah.

Poskim

Shulchan Aruch (OC 128:1): Birkas Kohanim is not with less than 10. Kohanim are part of the Minyan.

Bach (1 and Gra, DH veha'Kohanim): The Gemara does not explicitly say that Kohanim are included in the Minyan, but it follows from Sotah 38b. If there are only 10 in the Beis ha'Keneses, if they are all Kohanim, all ascend to bless. If some were Yisre'elim, the Kohanim would ascend and the Yisre'elim would answer. This shows that Kohanim are included in the Minyan.

Taz (1): Kohanim are included in the Minyan because they are included in the Berachah.

Mishnah Berurah (2): We learn from "I will bless them." Even if the majority are Kohanim, it suffices if there is one Yisrael to answer Amen.

Beis Yehudah (cited in R. Akiva Eiger): My Rebbi says that we do not do Nefilas Apayim (falling on the face during Tachanun), even in a fixed Beis Midrash, unless it has windows open to the Beis ha'Keneses. Some disagree and say that there is Nefilas Apayim in any fixed place to learn. This is wrong. Similarly, when there is no Sefer Torah, we do not read the Haftorah with Berachos (Levush 284), and there is no Birkas Kohanim. Once, the Radach did Birkas Kohanim without a Duchan and without an Aron Kodesh. This was wrong.

Mishnah Berurah (1): The same laws for joining to a Minyan for Kaddish apply here. See what I wrote in 55:33. Almost all Acharonim say that Birkas Kohanim is even without a Sefer Torah.

Mishnah Berurah (55:33): The Pri Megadim says that if one of the 10 is sleeping, he joins for a Zimun, just like the Shulchan Aruch says that he joins for Tefilah. It is not clear regarding Kri'as ha'Torah and Birkas Kohanim. Surely he does not join for Kri'as ha'Megilah, for then we require 10 to publicize the miracle, and one who is sleeping does not help for this.

Bi'ur Halachah (DH Ein): We require 10 because it is Davar shebi'Kedushah. The Ran says that Davar shebi'Kedushah is only Kaddish, Kedushah and Borchu. He learns from a Gezeirah Shavah, but agrees that it is only an Asmachta mid'Rabanan.

Chayei Adam (32:1): The Ran says that the Gezeirah Shavah is an Asmachta. I.e. Birkas Kohanim is mid'Oraisa even without 10.

Bi'ur Halachah (DH b'PAchos): Even if there were 10 at the beginning of Chazaras ha'Shatz, and some left, we hold that we finish Chazaras ha'Shatz, but Birkas Kohanim is forbidden, for it is a new matter. (This is why the Mishnah needed to teach that Chazaras ha'Shatz requires 10, and also Birkas Kohanim requires 10 - PF.) Chachamim merely fixed it in Chazaras ha'Shatz. If they began Birkas Kohanim with 10 and some left, they finish it.

Note: If there are not 10 listening to Chazaras ha'Shatz, the Berachos are almost l'Vatalah (OC 124:4), for one who stops listening to Chazaras ha'Shatz is like one who left (Drishah 124:1). If so, we may begin Birkas Kohanim only if a Minyan is listening. In pressed circumstances, we rely (that they are listening, to permit Birkas Kohanim - Doleh u'Mashke p.86, citing Ha'Gaon R. C. Kanievsky Shlita).

Kaf ha'Chayim (1): The majority must remain (in order to finish).

Kaf ha'Chayim (3): The 10 must be free males above 13 who brought two hairs.

Shulchan Aruch (25): If only Kohanim are in the Beis ha'Keneses, if there are only 10, all of them ascend. They bless their brothers in the fields. The women and children answer Amen.

Taz (22): The Shulchan Aruch did not say that they bless the women and children, even though it says that the women and children answer Amen, because Yevarechecha and all of Birkas Kohanim is in the masculine. The feminine would be (not "Elecha"), rather, 'Elayich'. The Berachah cannot take effect on children until they mature. Also, a woman seed is blessed only through her husband. Also children are blessed through their fathers. If so, the Berachah would be for the Kohanim themselves, and the Torah said "they will put my name on Bnei Yisrael." Kohanim are blessed through Hash-m - "I will bless them." Women and children can answer Amen.

Magen Avraham (37): The Shulchan Aruch did not say that they bless the women and children, for it says "so you will bless Bnei Yisrael", and not Bnos Yisrael. However, in Sotah we include converts, women and slaves. Do not say that they are blessed only together with men. If so, we should say similarly about converts, and if there are only Kohanim and converts, 10 Kohanim would have to (not bless and) receive the Berachah! (See the continuation of the Shulchan Aruch below.) Rather, women and children are not important enough to receive the Berachah themselves, like Rashi says about less than 10. In any case, a woman's seed is blessed only through her husband (Berachos 51b).

Mishnah Berurah (97): The Shali'ach Tzibur does not go up to bless, even if he is confident that he would be able to continue the Tefilah afterwards.

Kaf ha'Chayim (151): Some say that all of them go up, even though no one calls them to bless and no one is Makri (recites the words for them to repeat).

Kaf ha'Chayim (154): A minor should not be Makri. This is dishonorable for the Tzibur.

Mishnah Berurah (98): Even if there are no women or children to answer Amen, this is not Me'akev.

Note: The Shali'ach Tzibur may answer Amen only if he is confident that he will not get confused.

Shulchan Aruch (ibid.): If there are more than 10 Kohanim, the excess above 10 ascend and bless, and the 10 answer Amen.

Taz (23): Kohanim are important for the Berachah only when there are 10. Even one Yisrael is enough if Kohanim complete the Minyan. This is because Birkas Kohanim is in the singular. Kohanim are not in this category. We must divide them into blessers and receivers. They are considered receivers only if there are 10.

Mishnah Berurah (100): Since there are 10 who can answer Amen, it is best to fulfill both. Some bless, and 10 answer.

Bi'ur Halachah (DH Yoser): Even if there are 11, one blesses and the rest answer. Seemingly, it would be better for all of them to bless, for some say that Birkas Kohanim is only mid'Rabanan if only one blesses. However, the Pri Megadim says that in any case it is only mid'Rabanan when there is no Yisrael who tells them to bless. This requires investigation.

Kaf ha'Chayim (153): The Ari Zal says that Birkas Kohanim is one of the Tikunim of Tefilah. It is obligatory even if no one told the Kohanim to bless.

Kaf ha'Chayim (156): When 10 Kohanim remain and do not ascend, they should not call 'Kohanim', lest the 10 transgress.