We had a group of 21

last year. We ended up renting a super stretch HUMMER for our transfers to and from WDW to the port. We did have to have a excrusion follow us with all of our luggage...and there wasn't a whole lot of space left over inside the H2..but it was a blast. We went though KTS transportation. I dont' recall the exact cost but what ever it was is was way less than the Disney transfers per person and the price included the luggage rig.
This year I think we'll look into the "party bus". I think that holds more than 20 AND all the luggage. The party busses have the luggage area under the bus.

and the insides all decked out like the limos.