I'd say 1 or 2.
You can physically put more in the tank. Heck, you can stick 15 in there. In many cases, they'll thrash each other around, the weakest ones will die, their immune systems will decline, ammonia levels will sky rocket, and when it's all sorted out, there will be 0, 1 or 2 left.
So 1 or 2 is a good safe number.
OTOH I have seen a whole shoal of them happily swimming around together like a day care full of toddlers. But that was in a public pond that looked like it was in the 10,000+ gallon range.