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I consider those numbers minimums. More is much better, both for the chickens and for you. The higher the stocking density in a given area, the harder the chicken keeper's job to keep it clean enough for health.
Another thing to consider about the coop you're looking at is ventilation. A good number for that is 1 square foot of vent per chicken (so in your case 5 square feet), positioned so it can be left open in the winter without allowing drafts over the birds. If you live someplace with hot summers, you'll want additional ventilation down lower that can be closed off in winter.
Except if you live in a cold climate like I do. I have much more success with a coop 4 square feet per bird or (gasp dare I say it) LESS. The trick is proper ventilation. In a run, bigger is ALWAYS better no matter the climate.