I kept some of those in my 3 gallon once, and theyre so much fun!
Theyre quite small - a male grows up to 1inch, and a femaile grows up to 1.5 inches...
The amount? hmm not really sure - I would keep about 3-4
and if your keeping a couple of fish in there, i would keep 2-3 as a max.
if your keeping fish with them, give them somewhere to hide when theyre molting.