If a land hasn't entered the battlefield under your control during your turn by the time your end step begins, Zimone's ability won't trigger at all. You won't be able to put a land onto the battlefield under your control during your end step in time to have the ability trigger.

If you don't control a prime number of lands as Zimone's ability tries to resolve, it won't resolve.

In case you do end up with more than 31 lands, the remaining prime numbers below 100 are 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97.

There are a few more prime numbers, but we couldn't fit them all on the card. (Some of them would take up more room than we have in the text box by themselves, but you probably won't control that many lands.)