Every tank is different.
We have 8 of the Carinotetraodon travancoricus (pea puffer) in a 74 tower.
They are mature, but we'll see how much more as for the aggression gets.
They chase, but no one has been hurt.
Zero nipped fins.
They do not bother the 2 twig catfish or farlowella cats either.
Assassin snails (8-10) are all over the black sand and plow the substrate well.
The pea puffers kicked the snot out of the pest snails from any plants or driftwood we had from other tanks.
Feed the puffs live black worms for great results.
1 puffer is great, 2 are trouble.
-one becomes dominant.
A group usually serves well.
Your tank is small.
I'd just do one for now.
As they age, male and females are easy to sex.