My two current groups both have 7 players and the nights when everyone shows up... the combats definitely end up in the party's favor, much oftentimes to my chagrin. But now having done this through 5 different year-plus campaigns, all with 7-9 players... I've accepted what have become true facts regarding our tables when it comes to combat:
- The more players at the table, the longer each individual round takes. Because obviously there are more PCs, but also because I need to put more enemies on the table just to try and give a bit of combat parity.
- The longer an individual round takes, the more the players are exceedingly happy when their turn produces a substantial result. If they have to wait 20 minutes for their turn to come around again, they want to really do something cool with the turn they have. Which means me accepting and not getting bothered by my enemies dropping like flies. Because heaven forbid someone like a spellcaster uses their turn to make a Spell Attack and then botch their attack roll. At least the weapon-users usually have 2 or more attacks to do something in the round... that spellcaster who up-casts their one spell and then rolls a '4'? They don't even get the satisfaction of at least doing half-damage... they instead end up having waited an entire 40 minutes (on both sides of their turn) wherein they have done absolutely nothing. That is not something I enjoy doing to my players by any stretch.
- This has led me to believe that the true enemy during combats with 7+ players is not the monsters I throw down, but actually is BOREDOM. As a result... I accept that fights will really be steamrollers for the party because the challenge they all want is remaining engaged with the scene, and not specifically the fight mechanics itself. Especially when you add in the fact that with 7+ players there's going to be probably at least 3 or 4 characters that have healing at their disposal, so any damage I attempt to do to them to make the fights "challenging" are going to be wiped away every round anyways.
At the end of the day... interesting and compelling "combats" occur much more readily at my tables when I have people absent and there are like only four PCs. Because then I can throw more enemies down on the table so there will be more damage thrown about and more chances of really getting hurt (without it being wiped away so easily from massive amounts of in-combat healing). Plus the rounds will still go by fast enough so that each player can get several turns in pretty fast succession, thereby keeping them all engaged with the fights despite the large numbers of HP both sides have to burn through.
Given my choice, I'd prefer to just play with four people and 4 PCs because it allows for faster and more challenging combats... but I just have a lot of very interested friends who all want to play, so I feel an obligation to give them all chances to play. And I just I presume that my less-than-lethal combats at 7+ players does not actually bother any of them, since they all keep coming back to play in my games.