How many fish you can have in a tank has much more to do then with just size.
"inches" per gallon is a very relative recommendation.
Just as important are age of tank, compatabilty, need for free swimming room, and how much effort you are willing to take responsibility for maintaining proper water conditions.
If you are willing to due frequent regular water changes you can handle more fish then if you aren't.
Add them slowly (over months) and keep your eye on water params, they will let you know you have reached limit.
Your fish will let you know if they are in a "happy" environment or not.
