The first question is how much time will the chickens be inside the coop. Would they be confined for a weekend while you were gone, or for several days in bad weather?
The second question is what kind of bedding and coop maintenance method will you be using? If you're a scoop-the-poop every day kind of person then the coop can handle more birds. If you want to use deep litter method which depends on not being over saturated then 5-6 sq ft per bird will be necessary.
The third question is how much real space will the birds have, given the placement of your nesting boxes, waterer and feeder? Be sure to subtract if these items will take up floor space.
If you answered the questions in a way that would lead to less birds start with 4 because you know there'll be some other breed you just have to have and then you have room to end up with 6. If you answered the questions in a way that would lead to more birds start with 5 and exhibit maximum self-control so you only end up with 7.
Really, less birds with more space is just more pleasant. Otherwise you'll be one of those people posting, "help, my birds are picking at each other, what should I do?"