8 should be a good start. Chicken math strikes people differently. Last year I wanted 8, changed my mind to 10 at the farm, ended up with a free 1 (bakers dozen special) so total of 13. Due to illness and unwanted cockerels, I was down to 10 earlier this month. Then a couple weeks ago I got 8 chicks. So... it does happen.
Best advice I can give is build a coop that will comfortably hold way more chickens than you plan on getting!