Did you check to see if they are legal where you live?
I would say 3 max.
Maybe 5, but you are going to have to be really good with water changes, and there is no guarantee they won’t whittle it down themselves.
A few tips on them:
Its most fun (and cheap) to buy them as babies and watch them grow.
They grow an inch per month until about 6 inches, when they slow down greatly.
Keep them well fed to increase chances of not killing each other or anything else in there.
They will eat just about anything, but a pellet diet is best nutritionally. Most meat should only be a treat as it fouls up the water. You usually want to avoid live feedings.
They destroy plants, real and fake, less likely to attack the latter if well fed.
Prefer lots of cover and low lighting, they are less stressed this way and the lights hurt their eyes. You will see them more with more hiding places, because they feel more secure.
Don’t be afraid of them when cleaning the tank, they won’t bite and will play dead.
Enjoy!