I'd go safe. 1 for every 5 gallons. Make sure it's planted though. I keep 3 in a 20, and they still fight a bit, and I don't hold back on the plants!
Something you could go for are other freshwater puffers too, like irrubesco and South american puffers. They've been kept successfully with DP's before, but if it gets a bit hairy in there, you might need to remove 'em.