Seems like a simple question, how about a simple answer? Well, it’s not that easy. How many frontosas can you put in a tank is dependent on the size of your frontosa, your filtration, the upkeep of the aquarium, and how much you feed.
That being said, here are some things for you to think about. Fish grow and male frontosas can exceed a foot in length. Remember that when choosing a long term home, for even a single frontosa in a 40 gallon tank would be a minimum. Just because the fish lives in a tank does not mean it is healthy. It is a kin to old fashioned city zoos that have been giving way to more modern natural habitat zoos. Fish need room to swim, at least give them that. They are living creatures that are not here merely for our entertainment. Treat them with respect.
The higher the density of fish you have in a tank, and the more you feed, the better filtration you will require and the more frequent you will need to change water. No filter can compete with regular water changes. Skimp on water changes and a heavily stocked tank will suffer, while a lightly stocked tank may be fine.
In times of short power outages a lightly populated tank will be fine while a heavily stocked one may starve for oxygen. Keep that in mind.
There will be other hobbyists who are quick to brag about how many fish they can keep in their tank. Ask yourself: Have I seen their tank? Do they have fry? How long have they been in the hobby? Usually the answers are no, no, and not very long.
Imitate hobbyists who are experienced and whose tanks are doing well. Modify what they do to suit you, and your fishes needs