how many "you can fit", and how many you "should keep" are two entirely different figures.
the number you keep will be heavily influenced by your filtration and maintenance. without knowing the details of your proposed set up, you are looking at around 20
yeah if you overfiltered and did ALOT of water changes you would probably cram 30-35 in there but i would say get 20-25 and they'd be much more comfortable
Agree with everybody. Another way of looking at it is "as many as your filtration/waterchanges can handle". The more the better, because of "the nature of the beast".