The mob "Bat" is not affected by entity cramming. you can actully spawn unlimeted amounts of bats in one block space. as shown in the picture below, the bat can be attatched to the top of a block and it can be unlimeted of the same entity in the same block. This is a very serious problem because, when so many entitys are present, they can hurt the server and if someone were to destroy the block they were atatched to, they will all fly out and the server would hurt exstremly. if you try to kill all of them at once can that also hurt the server segnifently. please get this fixed before people starts abusing this to grief servers and destroy the expirience for others

If you want to test this by yourself, it is not hard to do. just place lots of bats with the bat spawn egg on the underside of a block