go with odd numbers, no more than 5 though. try to get 1m/2f if you can. they require brackish water probably depending on the type of puffer. Ottos would probably get destroyed. those little guys have a big attitude
Pea Puffer? Do you mean Indian Dwarf Puffers? If you are talking about IDPs, you can put probably 3-4, but Id stick with 1 Male, 2 female. Ottos would be killed though.
Brackish is salt added to the water. Theres different levels too, low and high end. Bracksih isnt a marine tank though, think of it as in between freshwater and ocean water.