Typically you want 2-3 females to every 1 male (at a minimum!), and it's going to be roughly 3-5 gallons per puffer, assuming you have a very heavily planted tank with lots of hiding spaces and clear territorial divisions.
So, you could get away with 6 females and 2 males, for example, as long as the tank was set up correctly. It'll always depend more on individual temperament though. Some pea puffers are more aggressive than others, and it's completely possible you'll get one that doesn't play nice with others. I've had it happen. For that reason, I like to keep them solo in smaller (5 gallon) tanks, anymore.
Keep in mind you will be buying small and unsexed most likely. So no male stripe on the belly and none of the females will have gotten big yet.
Buy extra and sell/return the ones that don't fit in you desired mix once you can sex them.
My luck of the draw has usually lead to many males and few females.
Are you already raising snails for them? They can go through snails at a terrifying rate. Have you got a worm bin in your kitchen to deal with veggie scraps and generate red worms? Peas will also eat red worms.
Some of mine have been willing to eat frozen but that can get expensive.
No one wants to starve their puffs.