There's two types of monster spawns, the ones that are set in stone, and the random ones. For the cow level, the cow king will always spawn, so that's set in stone, but everything else is virtually random.
Monster density is set in the Levels.txt. For example, the monster density for the cow level is 800, on all 3 difficulties. The problem though, is that I don't think anyone outside of Blizzard actually know what this figure translate to.
Levels.txt can tell us how many unique/champion monsters will spawn though. The Normal Cow Level can have between 0-1 "unique" groups, Nightmare Cows can have between 3-4 and Hell Cows can have between 6-8. By "unique", I mean unique and champion packs.
Also, the cow groups can be between 5-10 cows.