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!