9 gal. is plenty. I bought a 7.5 gal SS pot as my original pot back when I started brewing, doing extracts. When I went to make the jump to AG, some people said it was enough, some said no way. I'm still using it.*
*Life is easier doing 6.5 gal AG boils in a 7.5 gal pot if you just put in 6 drops of Fermcap-S (available at fine brewing supply outlets everywhere) at the start of the boil. No boilover worries, ever........