Join Date: Oct 2007
Location: Ontario, Canada
Depends on how much plants mass there is and how frequent water changes are. I'm sure you could stuff 50 of them in there, but they'd have no swimming space and you'd have to do water changes three times a day or something.
I like the consensus of a dozen or so. It's a good number for swimming space and school size!
I don't think cardinals are big eaters/poopers anyway, so they won't have too much waste. The inch per fish rule IMO doesn't work for planted tanks, or in general for that matter. It all depends on how much filtration you have (plants included), the number of water changes you're willing to do, and the fish requirements in terms of space (i.e. discus need tall tanks, schooling fish need long tanks).
Edit: I also agree adding a few at a time is ideal. That way, you get a nice idea of how many fish you think looks good. By the time you get past 12 cardinals, you'd have trouble telling the difference between 13, 14, or 15.