It depends on the campaign and level. IME, the number of combat encounters the PCs can handle increases significantly with level. In the apprentice tier, more than 4 combats was pretty rare, since there's not that much in the way of resource recovery from short rests (baring monk and warlock). Heroic tier tended to hit the 6-8 margin pretty regularly, but leaned closer to 6 than 8. Paragon tier generally had the same 6-8, because while the players had lots more resources, the opponents tend to dish out a lot more. The little bit of epic tier I've done allowed for a lot more, generally averaging 10-12 (although I ran an epic one shot that was over 20 encounters during the single long rest of the adventure).
Adventure theme makes a pretty big difference too. Dungeons and exploration tend to have more combats, while investigation and social adventures might have half a dozen for the entire adventure, spread out over several long rests.