Screamers only spawn more zombies when they are spawned by the heat map. Just adding them to biome spawns or BM groups doesn't give them the ability to summon more zombies. And also, during a BM all spawns other than the BM specific spawns stop. Which is why you won't get an organic screamer spawning during a BM even though you might be nonstop firing a weapon.
And if you mean more than 8 zombies per BM wave, there's an option for that in the game settings at the main menu. If you meant from a screamer horde, you'd probably need to edit those groups.