Ok, lets throw some light over the matter:
Each level has an ecology: a number of foes of each type. Every now and then the engine checks the ecology, and if it's below of the initial levels, there's a chance of a respawn. So it could NEVER be more critters than the starting number.
Also, the levels are quite 'small' so the provability of a bunch of respawns accumulating in a single spot is very low: the spawns are at random locations, and there's a low provability of not encountering them before there's more than, for example, two of them.
Said that, in the "user.cfg" you could use this codes (copied from systemshock.org): Monster Spawning
If you've killed everything in a given area, and you wait around long enough, you'll notice that more monsters are spawned to keep you busy. To minimize respawning, use the configuration variable: no_spawn
System Shock 2's ecologies look at how many monsters are in a given area. If a defined minimum number of monsters are present, there is a chance that a new monster will spawn. The variable: lower_spawn_min <n>
reduces the defined minimum monster count on all ecologies by <n>. raise_spawn_rand <n>
lowers the chance of a monster randomly spawning by changing the random chance from 1/i to 1/(i+<n>).
The link of the source and much more information: http://www.systemshock.org/index.php?topic=12.0