How Many Monsters am I Fighting?

The rules of DMing require that I be vague. Yes, I can be specific sometimes, but many times I cannot be specific. For example, if you are mapping a dungeon, I will be vague unless you take the time to measure every room you are in. I will be around the mark, but I will almost always be slightly off it. This also comes around when you guys are fighting monsters. Many times I will not bother telling you how many there are. Of course when the numbers are small (1-6) then I would probably tell you how many there are. I will also tell you how many there really are if you have the time to count all of them. However, if you are suddenly dropped into battle, or if there are far too many to count in one second, then I will use a vague description. As the battle goes on, you will figure out how many there are, but in a quick description of it's size I will using these vague descriptions.

Few: 1-4
Several: 5-9
Pack: 10-19
Lots: 20-49
Horde: 50-99
Throng: 100-249
Swarm: 250-499
Zounds....: 500-999
Legions: 1000+

I got these descriptions from a game. This game was one of my favorites when I was young. As such, much of my thinking has been altered because of it. I cannot guarantee that when using these words I am using them to describe the numbers listed here (for example, if you are attacked by bugs I might say "A swarm of them attacks..." even though there are only 25 of them), however I will use these numbers often enough that you should be able to recognize them. Again, as the battle goes on, you will be able to detirmine (with maybe a slight error) just how many you are up against. However, in the confusion of battle, you won't just know it off hand.

On another note, I severely doubt you will be fighting a crowd numbering greater than a horde. No matter what level you are and what you are fighting, it would be crazy to pit you against such numbers (unless, of course you CHOOSE to fight said numbers). I will try to use vague number expressions when describing armies (hundreds, thousands, ect.) Also, counting a size of an army is very difficult to do. In some cases you would have to do tricks, like counting campfires, in order to make a guess at how many there are in an army. You will pretty have to figure out ways of guessing army sizes yourself.