Albert Barnes Commentary Numbers 3:39

"All that were numbered of the Levites, whom Moses and Aaron numbered at the commandment of Jehovah, by their families, all the males from a month old and upward, were twenty and two thousand." — Numbers 3:39 (ASV)

twenty-two thousand - A number on which the commutation with the firstborn of the twelve tribes depends (Numbers 3:43–46). The actual total of the male Levites is 22,300 (compare Numbers 3:22, Numbers 3:28, and Numbers 3:34), and the extra 300 are considered by some to represent those who, being firstborn themselves in the tribe of Levi, could not be available to redeem the firstborn in other tribes. Others consider the difference due to an error in the Hebrew text.

The tribe of Levi is shown by this census to have been by far the smallest of the tribes.