How many puffers should I get?

I would prefer to keep only one or two Amazon or Congo spotted puffers due to their extensive dietary requirements. However, I've heard that both species prefer to shoal, and keeping them alone or in small groups may be detrimental to their well-being in the long term. Does anyone have experience keeping either species alone or in small groups? Additionally, if you've kept Congo spotted puffers, how much did they cost? I've heard they're expensive, but their price is gradually dropping as they're now being successfully bred in captivity.