You could prob squeeze 7-8 for bout 3 years then u could upgrade
bigger tank. Assumin all start at about 2 inch.
When any the indo gets to 12 inch it is then capable of killing smaller 6 inch fishes quite easily so may want to take care. I wouldn't say mature Indos (14 inch plus) have a tendancy to turn very aggresive. Though singly kept with another single predatory fish of similar size , say arro.... Then they may fight or at Least not get on.