They say 5 gallons per Puffer but I kept two in a 10 gallon before and one killed the other.
Plecos or small catfish like Cory's are fine with them
This is a great site.
http://www.dwarfpuffers.com/
Also its worth noting they don't eat flakes or pellets, you need to feed them Blood worms, Brine Shrimp and small tiny black pond sails that you sometimes see in fish store tanks. If you can't provide this diet daily, don't buy one.
Best of Luck
Edit: Don't listen to peter, Hes way off. Obviously has never kept them.
2 at the most in your 10 gallon tank. They are not brackish water fish either, although they can be acclimated to it, they are a pure freshwater fish in retail fish tanks.