We've done as many as 11 or 12 on a 4-cabin 48' cat. Mix of adults and kids. We usually had 1 or 2 of the kids sleeping on the galley bed (convertible bed/table). Works fine as long as everyone pitches in. Fridge space could be an issue, but if you have a couple cooler boxes for the drinks you should be fine. A desalinization water maker will make your job a little easier and your holding tanks will likely need to be emptied daily....every other day if you are lucky.
Have a great trip.