This is the mongen table you're looking at.
range - how far they spawn from the set coords. i.e. range 250 will spawn mobs randomly over a 250x250 square, the set coords being the center.
count - how many spawn at a time. I think this is limited to around 25. If you want more you'll have to add another row.
gentime - respawn time. i.e. how long you have to wait for a new mob to be spawned after it is killed.
smallgenrate - can't remember.
load - not sure
description - you don't need to edit this it doesn't do anything.