Preferred "combats per session" depends on the game played. Some games I've played (Paranoia, Call of Cthulhu) don't need to have any combat, though ONE is probably expected in a session.
If it's D&D (3.x, PF, or 5th, or another RPG based on these rules), Gamma World, MERP, Shadowrun... I'm happy with 1-3, though some of my players would probably like more. Regardless of game, I hate any combat that takes 30min+ to resolve, which I primarily see in later versions of D&D, particularly when I have most of my 6 player group available. (In these cases I wish we were still playing AD&D.)