Where it comes to parties fighting large numbers of monsters, we have a tendency to view these battles in terms of cold statistics only. That is, if ten orcs attack a party, we tend to calculate the effect of those orcs as so many chances to hit, such-and-such a chance to hit AC, a certain result of average damage accomplished per round. I say 'we' because I have done this myself.
However, where it comes to large numbers of creatures fighting a small party group, it isn't actually the creatures in combat that the party should worry about. The real danger is in the creatures not fighting.
Recently I had a group of nine player characters averaging 6-7th level, finding themselves in a fight with 250 orcs. They summoned themselves a host of goblins and grimlocks to help, but even so they were quickly overwhelmed. The orcs were supported by leaders of 3rd, 4th and 5th level, so it was a slog.
The room where they were fighting was a very large, mostly empty storage area for the orc lair (they had come in the back door), but it was narrow enough for the party to control a 'line' in part supported by stacks of boxes and barrels. For about twenty rounds, the party slammed against the enemy and failed to make very much headway; in all, they pressed about twenty feet forward. They could tell, however, through a weasel familiar who was scouting ahead (and avoiding detection) that there were at least a hundred more orcs in a large shop area connected to the storeroom. And these orcs were building a barricade.
After twenty rounds, the orcs still fighting the party broke. That is, the barricade was finished and they retired behind it. The party killed a few stragglers and then considered if they really wanted to continue the fight. In all they'd killed about 40 orcs.
The party didn't. They were actually looking for a dragon, which they later found. They went back the way they came, took another side route and the orcs boarded up the back door.
The real problem, see, was not the number of orcs or their hand-to-hand combat stats. It was the possibility of thirty orcs firing heavy crossbows every round, as these crossbows were loaded and handed forward in a continuous process. It was also recognized that in crossing over the barrier, the party would have had to risk as much fiery oil as the orcs may have been prepared to throw into their mostly empty storehouse. Plus the tribe had a few mages and clerics, who were very difficult to even find among the host.
Another point the party did not consider - how long would it take a hundred orcs not involved in combat to quickly lay down a bunch of easy-to-employ traps, giant rat traps for instance, the placement of spreads of small caltrops or some kind of snare. Suppose two groups of ten orcs take off, find some long benches from the mess hall and then use it to take a run at the lead fighter and his mate? The party finally climbs over the barricade and all at once the orcs split apart and bang, mess hall bench right in the tenders! How much damage does that do?
It isn't just that the number of defenders increases their chance to hit; it increase their chance to think - a far more dangerous opportunity for an attacker to risk giving.