It complete depends on your engine and pipeline... i did write a bloody long tutorial and explaination for all of this back when this forum was first started, perhaps I should again and have it sticky-ed.
Effectively speaking you have to work around the limitations of your game.
Using a Matrix for terrain and the built-in fog you could probably get around 20x 1,200 polygon characters on-screen at once on a mid-range machine at a healthy speed.
If you decide to use Pixel Fog and add a Shader Bloom/Light Accent to the scene... this will no doubt effectively cut in half the amount of polygons you can use at the same speed.
The same is said with Dark Basic Professional that, having 20 monsters of 1,200 polygons is like having 1 with 24,000 polygons ... but this isn't true. It is actually faster to have a single model being rendered than it is to have a whole pipeline of them. As such you could probably make a boss with slightly more polygons than you could with a group of monsters.
I think i'll work on this tonite actually, come up with a guide for game media creation... this question is becomming all too frequent.
