Seems a bit unfair that only either the smallest numbers or the numbers with lots of divisors get their own door sprites. I mean, you can't give sprites to every single number, there are too many. but on the other hand, it's nice to be remembered.
You could probably add more locks onto a 2x2 door, but I think this is the extent of "comfortable" fitting keyholes. Go to 2x3 size.
Also, 24 doors would be more readable as four 6-lock flowers.