Assuming you're talking the housing part of the coop only, not the run, a max of 30 IMO, and because you guys can get some pretty severe weather (I'd think), I'd go with less than that, since your birds will probably spend a decent amount of time indoors this winter...More space is always better.
How many can u fit? OR How many would u want if u were a chicken? I try to think of quality of life, Im not an extremist by any means, but I like my chickens, even my meat birds to have a little space. AND after watching an episode or two of hoarders, I want my animals to live comfortably. I have been to coops on farms that had a sea of moving chickens at my feet and I had to shuffle instead of walk correctly, they were all healthy and well fed, but i dont see how u can enjoy them or have them enjoy themselves like that.
Before I would try to answer that, I'd need to know what your outside space for them looks like and a bit about your management techniques. I'm assuming you are in the Grand Rapids that is in Michigan. That plays a part too.
The general rule of thumb used on this site is a minimum of 4 square feet per chicken in the coop with a minimum of 10 square feet per chicken in the run. This is intended to cover people from Fairbanks, Alaska to Miami, Florida, from Lima, Peru to Perth, Australia. There is a tremendous amount of variation in what will actually work because we all have a tremendous amount of differences in our specific circumstances and most chickens are pretty adaptable.
Chickens do not like snow and wind but mine don't mind the cold, at least down to single digits Fahrenheit. That's the coldest I've seen it here so I don't know if they will go out and play if it drops below zero. If you are in Michigan, you are going to have some snowy windy days this winter where they will stay inside. That argues for more space inside. But if you build at least part of your run to block the wind and keep it clear of snow, that greatly reduces the pressure on coop space.
Weather is not the only thing that may keep them inside. You may plan on keeping them inside for extended periods of time anyway for whatever reasons. In that case, I'd argue for more coop space per bird. You may be set up where they are able to get outside into a very large useable space every day, in which case you can get by with less inside space. But I suggest you plan on the worst case scenario, not assumning nice weather or perfect conditions every day.
Some people do quite well with 2 square feet per chicken in the coop. This is where the coop is used only as a safe place for them to sleep at night and a place to lay eggs. Otherwise they are outside all day every day. Some people have trouble with more than the 4 square feet per bird, say where they are confined for great periods of time. There is no one right answer for all of us. If you can follow the 4 and 10 rule of thumb, that is a great starting point. If you don't follow that, you have to make adjustments.
Just because you can squeeze a certain number of chickens in a space does not mean you have to. Determine what your goals are and if you can meet your goals and provide extra space, you are doing great. Hope this helps. Good luck!