As long as you are not on the Grand Plan, when you book a Fireworks Cruise the boat is yours (along with the provided driver). So, it's totally up to you how many guests are on the boat. At the Magic Kingdom Fireworks you may get a smaller pontoon boat with a party of 6 or smaller, at IllumiNations they only run the larger pontoon regardless of the number of guests.
With the Grand Plan (at IllumiNations), all guests are assigned to one pontoon, so you will be seated with other guests. I am not quite sure how Grand Plan is handled for the Magic Kingdom fireworks since so many more boats can view the fireworks and since there are 4 marinas capable of sending out boats.