A Dungeon Master has many options and many decisions to make when designing combat encounters. One such decision is about how many enemies to include in an encounter. Large groups of enemies can give a feeling of tension while also making the players feel powerful. However, there are some possible issues with the approach. Rolling can take a while, and it's possible to end up with a combat encounter that isn't all that different than fighting one enemy with many attacks, among other things. Having seen many such situations, I hope to get my thoughts on the manner recorded. It helps me think through the topic for my own benefit and hopefully someone, somewhere, finds it useful.
We could approximate some large groups of enemies by using one that has multiple attacks. As it takes damage, it gets fewer and fewer attacks until it finally dies. Now, that can make for an interesting single creature encounter. The issue I see, however, is that in these cases there often isn't much of a difference. It would be best if when we use one or the other, there is a marked difference. The one difference that does exist is mobility. Many enemies allow you to restrict movement and also to position them in ways that threaten multiple people. One enemy fighting a party is the opposite situation. It is your single enemy that is or can be restricted.
There is a lot of potential for enemy movement and clever use of positions when many enemies are encountered. Some could try to go into tactical positions, block access to certain routes and force players to hack their way through, or go around and flank. They don't all need to move in the same way either. Some can attack at range, some close up, some could be flying, as well as many other options. This is one of the major differences from using one enemy. It also means that multiple people can be engaged while also avoiding opportunity attacks. If it was one creature attacking everyone in the party, it would need to move between them and possibly trigger an attack from every one of them if using melee. It's a bit different with ranged attacks but we could still get into such a situation when cover is used. It would also tend to go after one goal, where a large group lets you go after 2, 3 or even more. I find that a static encounter is what we want to avoid here. For more on that topic, you can check my other post here. That one was about confined spaces but the central issue is being locked down in an encounter in both cases.
Carving a Path
When surrounded and forced to move, not every creature necessarily needs to be killed. Also, not every goal needs to be chased. The players could pick one and pursue it fully. They could also choose to escape. In this kind of case, hacking their way through to escape is a valid option that isn't otherwise available. A crowd of weaker enemies, however, makes this a tactical option. Where this can get a bit tricky is when movement is included. How does it interact with their attempt to hack through? It's also worth noting that moving through like this triggers opportunity attacks as well. And with a large crowd, it would be quite a lot of them. This makes it a rather risky tactic, though in such a situation risky may be better than certain death. It's part of the reason why them being weaker is necessary. It tends to work best with creatures that fall from a single attack or at least have a good chance to. Player multi-attack, area of effect spells, or other means to attack more than one creature allow the players to go through more than one square at a time this way.
There is another issue when trying to do this. When they break an opening, initiative order and movement rules can result in a case where the crowd just catches up to them and tries to swallow them again. There isn't much of an easy solution in these cases beyond moving away from combat at that point and handling it as a chase. However, it can sometimes work out anyway. It partially depends on initiative order. If it happened just right, the enemies could end up blocking their own from moving and allow the party to get a good lead on most of their pursuers. This was a rather cool situation in a game I was a player in. However, it was probably largely due to luck on our part and also because zombies don't get as much movement. In our case the party outpaced the entire group of zombies except one or two. The other thing to keep in mind is that it would take the enemies more movement to move around the front of your players. Of course, it also takes the players some of their movement to escape too. The deciding factor then comes down to terrain features. A choke point after they break out can ensure they no longer get surrounded.
Too Many Layers
If we have too many layers of enemies, it's theoretically possible to create an encounter where everyone is stuck. It is sort of like the stalemate situation in the Carving a Path section above. The difference, however, is that in this case escaping may not be the goal of the players. However, this can still be a problem because it restricts movement and therefore tactical choices. You probably don't want to lock down all your players in a large crowd of enemies that they can't ever break out of. For this reason just keep in mind how many layers of enemies you'll be throwing at your players. There is such a thing as too many at once and I believe this is when it happens. To avoid this but still throw large groups at your players, you'll probably need to break them up somehow.
Break Em Up
Having many enemies allows you incredible freedom in deciding how many will arrive and when. They all don't have to arrive in a massive crowd. The encounter can be in multiple waves, or at least what seems like multiple waves due to initiative order. This allows a turn or two for players to strategize, get better positions, pick off a few at range, maybe have a couple rounds of skirmishing, and do whatever else they need to do.They might still be able to see all the enemies at once. It just might take some of them 3 turns to arrive as opposed to 1.
Of course, having more enemies means you have a chance to give each one their own identity and character. This may make some of them more obvious targets. It also gives things more personality and allows some of them to get killed in combat while others retreat. The impact of this won't be combat related or tactical but it's important all the same. Some could even turn on each other mid-combat if things aren't going well.
Rolling A Lot
Many enemies tend to mean a lot of rolling on the Dungeon Master side. It also isn't as easy to handle as other situations. Rolling many dice at once is a common way to speed things up but we'd need to think of ways of assigning the dice to players. If you have 4 or less, having different colour dice ready to go works well. Each colour corresponds to a player so after you roll, you don't need to decide which dice corresponds to who. This way you aren't playing favourites. Of course, you'd need to plan and prepare the dice ahead of time to make sure you have enough. Otherwise, you'll be taking quite a bit of time rolling. Rolling a lot of dice in these cases is unavoidable, just like with enemies that have many attacks. However, how we actually roll them can help save time.