What is the significance of 212?
Malbim: There are 30 for each day of the week 1 - 24 for the 24 Mishmaros, and six appointments over them - the chambers (verse 26), the storehouses (26), Klei ha'Avodah (28), Klei ha'Kodesh, flour and wine... (29), the Chavitim (31) and Lechem ha'Ma'areches (32). Alternatively, one was appointed over Klei ha'Kodesh, another over flour and wine, but no one over the Chavitim.
Malbim (25): There were another two - one was over the gatekeepers, and one was over the appointees. In all, this is 212.
What do we learn from "v'Chatzreihem Hisyachsam"?
Redak: They were in the Chatzeros that they received in the days of Yehoshua. Each traced his lineage to Beis Avosav to know in which city they dwelled, and to come to Yerushalayim in the known Mishmaros, like was enacted.
What is "be'Emunasam"?
'Rashi': Their continuity. Radak - this is like "u'Ma'amar Esther Kiyem." we find that Emunah refers to continuity - "va'Arastich Li ve'Emunah", "v'Hayah Emunas Itecha"... David and Shmuel established Mishmaros of Kohanim and Leviyim. They did not serve all at once - refer to 9:25:2:1.
Radak citing Yerushalmi Ta'anis 4:2: There was a great Umnus (skill, to fix 24 Mishmaros; according to rotation of serving), no Mishmar will [serve on Yom Kipur of Yovel and] receive inherited fields [that were Hukdash and not redeemed] and repeat [until every other Mishmar receives].
