I've always thought (whether I'm correct or not, I'm not sure) it was 5% of capacity maximum, or 3000 if 5% is more than that. eg 5% of 75000 = 3750, so it'd be 3000 there, but in a 25000 stadium it'd be 1250 max
10% of capacity or a minimum of 3k seats for stadiums with a capacity exceeding 30k. I think 3k is about the maximum allocation you get anywhere these days in the PL give or take a couple of hundred.
source?