[Delphinus]

This is my take. Others may give you differing advice but this is what I believe.

Two is OK if you have a male and a female. (You can tell the genders apart by the jawline - males have a bulldog appearance - they carry the young). Any more than 2 and the odd man out (or girl) tends to be bullied.

I'm not 100% sure about two of the same gender. Personally I wouldn't chance it.

Now, here is the wrinkle with the "1 male, 1 female" idea. They breed constantly. Cool right? Well, maybe not so much. The male carries the babies, and won't eat while carrying babies. Once the brood is expelled, they "go at it again" and before you know it, the male is carrying again. Males usually die from exhaustion/starvation because of this. You should try separating them so the male can regain strength. Basically, without intervention, a mated pair becomes a single female sooner or later.

One by himself or herself is obviously OK from a territorial or "too much spawning" perspective. However, both times I had a single female, they eventually died and I think they may have been eggbound. Basically, without a spawn to release the eggs, eventually a female explodes. Not 100% certain about this but "2 out of 2 times" is hard to ignore.

Fun stuff eh?