18 is about right for me, well I have a little more then that.

5 dogs:
happy
misty
shadow
perky
Ripley

11 guinea pigs:
missy
friskit
patch
smores
poppy
smudge
spud
blizzard
blondie
puddin
checkers

1 Rabbit:
kari

1 betta:
fisky blew/blue bubbles

2 large aquariums with several fish in each.