[NCASAUmp]

Speaking ASA:

3-6: Sleeves or straps of the uniform top may be adjusted, with or without tie-ups, to the comfort of the players, provided uniform numbers remain visible.

3-6-D: An Arabic whole number (0-99) of contrasting color, or outlined in contrasting color, at least six inches high must be worn and visible on the back of all uniform shirts. No players on the same team may wear identical numbers. Players without numbers will not be permitted to play. There is no penalty for a player wearing a wrong number or a duplicate number. Correct the number in the score book or have the player(s) change jerseys in case of duplication, and continue play.

4-1-A-1: (Place in context of the line-up inspection) If a wrong number is on the line-up, correct it and continue playing with no penalty.

And then, there's the Blood Rule...

4-9: ...Uniform rule violations will not be enforced if a uniform change is required...

The whole point is to encourage participation, not to get tied up in a nit-picking battle over something as simple as a number.