Why is the word "Atudim" in Nachshon's Korban written with a 'Vav', and regarding the other Nesi'im it is without a Vav?
Ba'al ha'Turim: Because David, who will descend from Nachshon, will be blessed with six (the Gematriya of Vav) Berachos. 1
Da'as Zekeinim: It hints to six Tzadikim who will descend from Nachshon, and the 'Vav' in "v'Korbano" is to prevent Nachshon from becoming conceited (refer to 7:13:1:1,3).
Why did he bring two bulls for Shelamim?
Rashi (23, citing R. Moshe ha'Darshen): Corresponding to Moshe and Aharon, who made Shalom between Yisrael and their Father in Heaven.
Ramban (2, from Bamidbar Rabah 13:14): It corresponds to David and Shlomo, the first kings of the dynasty of Yehudah, who were both Tzadikim, and the Malchus was established.
Da'as Zekeinim: They represent the two Luchos. Shelamim represents the entire Torah - "v'Chol Nesivoseha Shalom" (Mishlei 3:17).
Why did he bring rams, goats and lambs for Shelamim?
Rashi (23, citing R. Moshe ha'Darshen): Three species, corresponding to Kohanim, Levi'im and Yisre'elim, and to Torah, Nevi'im and Kesuvim.
Why did he bring five each of rams, goats and lambs for Shelamim?
Rashi (23, citing R. Moshe ha'Darshen): Corresponding to the five Chumashim, and the five commandments that were written on each Lu'ach.
Ramban (2, from Bamidbar Rabah 13:14): They correspond to the 15 kings from Rechav'am till Tzidkiyahu. Some were complete Tzadikim, some Beinonim and some complete Resha'im.