This is the best info I could find on Cooks reading through the boards:
A good estimate on cooks is 10 cooks for every 100 troops you send. The great thing is that the most cooks you should ever need is 30, because at 30 cooks you have maxed out any bonus they give you in battle. 30 cooks should keep any size army at 100% morale during a battle. And they are nearly incincible to destroy. The bad thing about them is that they are soooooo expensive to make and maintain.
If anyone finds better information on this, please post it here.