As others are saying, keeping dwarf puffers in groups can be tricky. One thing that can work is keeping them in large groups, say six or more. As with other mutually aggressive fish (e.g., cichlids, archerfish, mudskippers, halfbeaks) this diffuses out the tension by preventing any single specimen from becoming hyperdominant. It also makes it difficult for one to hold a territory for too long.
HOWEVER, deliberate overcrowding relative to the size of the tank has risks, and should only be done by experienced hobbyists. The problem is that keeping a lot of fish together in a small space overloads the filter and the life support capacity of the aquarium. Pufferfish are extremely intolerant of poor water quality and low oxygen concentration, so keeping, say, five specimens in a ten gallon tank might work from a behavioural point of view but end up being unstable in terms of the health and longevity of the fish. Keeping six specimens in a 50 gallon tank, on the other hand, might work quite well, and give you plenty of scope for adding other species of pufferfish as well.
Cheers, Neale