How many Doctors do I need?

This is the best info I could gather from reading all the boards on the subject of doctors:

1. Doctors only recover hit points, they do not revive dead units. And they do so only at the end of each round of combat, so if a unit takes enough damage to die in one round it will not have recovered his life by a doctor.
2. Each physician retrieves about 8 hit points per round.
3. Doctors have minimal effect mechanical units (I am still not sure what "minimal" is).
4. 20 doctors is the most you should ever need for your army when attacking someone. From everything I have been able to find, more than that does not increase their effect.

Again Doctors are really expensive, too. So don't go bankrupt trying to make and maintain and entire army of them.

If anyone else finds better info, please fell free to post it here.