It depends on the size of the fish. The general rule is "an inch of fish per gallon of water," meaning if you have a 10 gallon tank, you can do one 10 inch fish, or ten 1 inch fish, etc.
One small fish per gallon initially. Adding thrm in small groups every couple of weeks. Oncd uour aquarium is mature and your skills ste honed you should be able tj handle two neon-tetra size fish per gallon.
All depends on fish type, size, tank, etc.
Four at the most.
If definitely consult various places
You should Google it. I wouldn't trust Petco to give you a correct answer.
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