We chartered a 4700 in the
BVI, and had 4 of us that dove, with 8
tanks on board. The rental company provided us with tank racks that we tied to the rails on the stern of the
boat, it worked perfectly. I'm sure you have been into it, but DO NOT MISS the Rhone--it is incredible. If you get to the dive site a little after it gets light (we did) you will be the first ones on it, and there will be
fish everywhere. Also, if you hit the
mooring buoy that is the only yellow one in the middle of the red (dive
charter boat) buoy, it will put you directly over the top of the bow.
We rented our
gear from UBS dive center (they have a website). It was OK
gear, but not excellent. It was well worth the
price to have the gear on board for whenever we wanted to use it. Air fill stations are all over the BVI, and they gave us a book with locations of all the fill sites. With 2 tanks each, we never had any problems running low on air.
To further answer your question, we would have had space for maybe a dozen tanks easy, maybe 16 without crowding too much on the 4700.
PM me if you want, and I'll send you a link to some of our pictures of diving in the BVI.
The diving there is truly world class. Enjoy!