Gail Damerow (author of Storey's Guide to Raising Chickens) suggests a minimum of 4 square feet per hen in the coop, and at least 6 square feet per hen in the run. So at least 10 square feet per hen total.
I have an 8'x8' coop as well, with an attached 8'x10' run. My chickens free range every day over many acres, except for some snowy days in the winter when they prefer to stay inside.
Based on Gail Damerow's numbers, an 8'x8' coop should be able to comfortably house 14 or 15 birds (taking into account the wall thickness). Admittedly, I do often have 20 or more birds in that coop (chicken math, LOL). Currently there are 17 in it. I can tell you that over the years I have noticed that the less crowded the coop is, the more eggs I get per hen per week. And vice versa. So the feed cost per dozen eggs is definitely less when the coop is less crowded.
So really it is up to you. Any hens with mean or aggressive temperaments will be accentuated in closer quarters, so be sure to keep that in mind when selecting chicks. So for example, I wouldn't suggest keeping a more assertive breed like Rhode Island Reds with a very docile and sight-impaired breed like Polish, because the Polish will get picked on. Try to pick breeds with similar temperaments, or get chicks that are all of one breed. If you pick similar breeds, and give them a big run, I think it should work fine, especially if you can give them a covered area in the run so they don't all dash for the coop when it rains.
