Vastly system-dependent question. In 5e D&D, my ideal would be one to three fairly 'simple' fights (not very challenging, low stakes, there to burn a few resources and to build tension) with one or maybe two 'centerpiece' fights (more dangerous, more tactically involved and complex, higher stakes).

For my current favorite system of choice, Warhammer Fantasy Roleplay, the formula is different. Combat is brutal, takes longer in terms of overall number of turns, and there's almost no such thing as a 'low-stakes' fight; a scrap with a small goblin patrol can be a life-or-death affair, and injuries are hard to recover from. With combat being so involved and high-tension, and more emphasis on non-combat sequences than D&D, one combat per session is plenty, more than two starts to be too many. That said, combat is still sufficiently central to the system that less than one fight for every two or three sessions is too few.

On the even more extreme end, something like Call of Cthulhu might well involve as few as two or three fights in an entire campaign. That's a game system where combat is almost more of a failure state than an intended piece of gameplay.