twenty to the dozen

a dime a dozen

So common as to be practically or almost worthless.

baker's dozen

Thirteen; a group of thirteen.

baker's half dozen

Seven, a group of seven.

banker's dozen

Eleven.

cheaper by the dozen

Handled more efficiently as a group, rather than individually.

decimal dozen

Ten, a group of ten.

six of one, half a dozen of the other

The two alternatives are equivalent or indifferent; it does not matter which one is chosen.

twenty winks

A very short sleep, half as long as forty winks.