Battling multiple monsters

[Blazeguard]

I tried searching but couldn't find any posts regarding this.

When entering a location such as 4. The Mine in the Gold in the Hills adventure, how do you account for and manage all the monsters in that room?
Do you limit how many can attack at once based on reach, if the adventurers are in a doorway, etc?
What other factors do you take into account?

Even with retainers, 18 - 21 goblins/hobgoblins would make short work of a party if they were all able to attack each round.

[Dimirag]

I tend to not allow more than 6 opponents against a single target (fully surrounded) but make mods on the fly, lances and similar weapons allows for a second line attack if the monsters are trained to fight that way.

I divide the monsters into groups and use those groups as units.

[black1blade]

There are several ways to think about it, I just think about the typical 5 foot grid albeit loosely especially with large spaces.

[Solomoriah]

I always use a battlemat and miniatures, save only for those times when little fliers like stirges are involved.