Have a look at Andre's website in the gallery:
You should get an idea of the amount of people vs boat size.
I'm not pushing a Parker rib but he does have loads of pics which are worth looking at to see how other companies carry passengers.
As Searider says, luggage storage must be taken into account if they are turning up with suitcases....
Also make sure you find out if you need suspension seating for your working crew / passengers as I have no idea in your area.
If I'm honest I'd be looking at a 9m