I wouldn't dare stuff even my 20 girls in that small of a place. . . Especially if it was too low for me to walk through, but I like to walk into the coop. I hear it is about 3 sq ft per bird, but it could also depend on the owner's preference.
Right now my 9 chickens stay in a 5x6 coop that sits above a 5x12 run but they are free ranging right now. When i finish my new coop i would like to use this smaller coop as a breeding pen or for broody hens.
That's not bad. The reason I do mine that way is they have plenty of room to range and basically just sleep and lay in the coop. Other than that, they're almost never in there. SOOOO, that being said...if that's what you're doing too, I see no issue with 20 in an 8x8, but I honestly wouldn't go more than that. For 18 I'd say it would be adequate for those circumstances.