Is the tank already cycled?
Yes: ammonia and nitrite read zero
No, or I'm not sure
Fish already in the tank?
Yes
No, this is the first group
Do you have a spare tank for quarantine?
Yes
No
New fish to add:
6
Size of the fish
Small (tetras, guppies)
Bigger (goldfish, cichlids)
A cautious planning rule of thumb, not a vet's prescription. Every species and tank is different: check how many your species and your tank size can really hold.