There are different sizes of crate and there are different sizes of eggs, some crate can accommodate more eggs than the other.
For example, the crate used by farmers; you will find 100 eggs, which are 5 flats of 20-count eggs or 180 eggs approximately 15 dozen, which are 6 flats of 30-count eggs.
The structure of the crate is built in such a way that the eggs have space and will not crack during transportation.
However, the market size of a crate of eggs for sell, contains 30 eggs maximum. The eggs can be big or small, attracting different prices.