Hi, your in the right place for info. I'm gonna say you need a min of 20 gallons for 12 tetras like fish. The longer the tank the better and a it depends on species, like I would do 15-16 of flames in a 20g but I would not even do 10 white skirts on a 20.
If you Can get your hands on a breeder tank, they are typically longer and allow more stocking room.
36 gallon fresh water