I don't know about the sexes on how many but usually 5 gallon per fish is the rule for pea puffers because of there territorial nature. 3 gallon per fish is also ok but 5 gallon is best. So you can put 6 pea puffers in your 29 gallon easily. Since tank is already planted so they will do good and won't have big aggressions because line of sight can be avoided by plants.
Puffers are really cool. You'll be surprised to know that your 29 gallon tank is actually less in volume if you measure it's dimensions. My 29 gallon tank wound up only being 26 1/2 gallons without gravel. The best suggestion I would give you for puffers in that size tank is 1 male and 2 females. Puffers that are omnivorous (and I'm not sure about your species) may wind up grazing on your plants. And they will need meat too such as frozen blood worms, small prawns and the like. They don't take well to flakes and pellets... Especially while their becoming used to their new home. They also like crunchy lettuce and peas. Even with just 3 in the tank, there will be plenty of movement. When they get used to you, they'll come to the front of the tank to look at you...not just because they think they're being fed. Good luck.
