As many as you have space for, according to GAAMPS.
Are Roosters Allowed?
Is Permission Required?
Any Coop Restrictions?
Refer to GAAMPS. Basically, don't hurt your birds with their housing.
No matter what part of Michigan you live in, you ARE allowed to keep chickens (or any farm animal at all, for that matter). Don't let your city tell you otherwise. State law supersedes city law. If you're asked to relocate your chickens, print out the pages from the links below and fight it. Supreme Court has ruled in favor of the farmer time and time again in circumstances such as this. Chances are, if you know what you're talking about, they won't hassle you anymore; just make sure you're adhering to the GAAMPS (not difficult).