Only thou shalt not number the tribe of Levi, neither take the sum of them among the children of Israel:
Numbers 2:33
But the Levites were not numbered among the children of Israel; as the LORD commanded Moses.
Numbers 26:62
And those that were numbered of them were twenty and three thousand, all males from a month old and upward: for they were not numbered among the children of Israel, because there was no inheritance given them among the children of Israel.