Bryond - As soon as you go to 6 opponents, assuming you're playing by the book and not fudging, you are so far down the EL/CR scale that it's not even funny.
Look at an EL 10 encounter with 6 opponents: 6xCR 5 opponents. Now, assuming for a second that we're going to go EL par, so this should be a standard fight. Not too easy, but, no one should die either.
6 CR 5's. Let's see, that gives us a choice of: basilisks (with the lovely save or die mechanics involved - 6 saves per PC per round=instant TPK, from an encounter that should be non-lethal), but, let's use manticores. One of my favorite critters.
Let's see, we've got two claws at +10 and a bite at +8 or spikes at +8. 10th level PC's are going to be running in mid-twenties AC, very easily. The melee types are likely hitting 30 with a buff or two. Even the rogue is well into the twenties. So, our critters have about a 25% chance of hitting. At best. Likely, it's closer to 10%, particularly if the party has, in Reynard's view, ample warning of what's coming.
Best option here is 36 tail spikes. About 3 hit doing 3d8+6 damage total.
The wizard isn't even feeling that.
Can it be done? Oh probably. I'm sure most things can be done if you massage the numbers enough. Can it be done easily? Not a chance. I watched far too many combats go this way - lots of baddies, the PC's basically ignore them as they mow their way through. The baddies just can't hit, and, even when they do, don't do enough damage.
Note, that's with 6 baddies. Pump the numbers up to 10+ and watch what happens.
About the only way large groups work is if they have special attacks, like our basilisks above. If they are straight up melee monsters? Not a chance.
What blows my mind is that people still remain convinced that 3.5 did this well. Look at pretty much every module out there. You will see almost no encounters with more than 5 baddies. There's the odd one, usually when the baddies have some sort of mount, but, by and large, 5's the max. The math just fails beyond that. It fails because the power increase per level is not flat, it's exponential. Going from 2nd to 3rd level is NOT the same amount of relative power increase as going from 10th to 11th. Maybe for the non-casters, but, the casters? They are getting new levels of spells, plus gaining more slots of their existing levels. Plus the rapid increase in PC wealth. Plus the feat synergies. Plus, plus plus.
Are the absolutes wrong? Maybe. Fine. I'll concede that. But, at the end of the day, pretty much every D&D designer backs me up on this. Module after module reflects this understanding. Whether it's Goodman Games, Paizo, WOTC or Bleeding Edge - they all follow the same paradigm. 5 or less.