<<Can we motor safely with 8 or 9 or more adults on board?? >>
I'll submit that you're inviting trouble. Motoring only, on calm water, for a short evening trip, you'll probably be OK. But raise the sails, get a thunderstorm, rough air or water, or a couple of drunks among them, and you've got problems.
When planning for aviation, the official "guesstimate" for a person's weight is 180 lbs. There has been talk about raising that to 200 lbs. in America because we're all so overweight. If you have 200 pounds of sails, anchor, battery, stuff, onboard (you've probably got more than that); it leaves room for five 200 pounders. Overloaded boats are a primary cause of marine accidents.
Personally, I won't consider more than four on board -it's just too crowded to be enjoyable.