How big of a breeding group of mice can I keep in a tub that is 16 by 12?

I usually kept mine in 1.4 colonies. 16 x 12 should be plenty of room for a colony this size....

I keep mine in groups of 1.4 as well.

I keep 1.5 in 22 x 16.

16 x 12 should be ok for 1.3 or 1.4 but I wouldnt go more than that personally.