Should have? As many as you want.

How to use? Read below:

Berserkers are a set-off weapon, once they're charging they will never stop (Unless scared to the "Wavering" level, and that takes a lot). And for that, the best time to set off is when your infantry line engages the enemy's. Also, a good think to note about them:

If you set off a zerker unit behind a semi-scared enemy one, consider than unit routed. One more thing to keep in mind, they will go for the closer enemy unit till it routs/dies. So, don't set them off near enemy's archers or they will keep chasing that unit.