the more filtration the better, and its never a bad idea to have more than one, but for a 20g tank I don't think it is necessary. Personally, I would just get one, that is rated a bit bigger than the tank you have. Also, more filters does not equal more fish, if someone gave you that idea
!!! Depending on what kind of tetra you get, I would say for the smaller species you could do 12 or so of one kind and a few guppys. Consider only getting one kind, they will school much better. If you get several different kinds and just 3 or 4 of each chances are they will not really school, just more or less swim around however. There is nothing wrong with that, but consider how you want the tank to look!
Good luck
Good luck