As ggunn mentioned, for most shops it's based on demand. Ask the shops you're interested in, but if you have 4 divers that's enough, and with some shops, 3 divers is enough, to "make" a boat. I've done most of my diving there with Deep Blue. They leave from the downtown Aldora dock and pick up divers at their hotels on the way out to the reefs. The last time I was there we had 4 divers in our group so we went out any and every afternoon we wanted. After the second morning dive, they'd drop us off at a beach club where we'd have lunch. They'd pick us up at the beach for the 2 afternoon dives and deliver us back to our hotels at the end of the day, unless the weather got iffy in which case we'd all go to the marina and take a taxi back to the hotel/s. The dive shop covered the taxi fare back to their shop downtown, or the hotel, though the taxistas can be hit or miss in terms of accommodation. (About half want to charge for every hotel drop off. The other half charge one fare between the marina and the shop, and they just drive to the shop and collect it even if they don't have anyone to drop at the shop.) It would depend on the shop, but you might be able to go out in the afternoon with just 2 or 3 divers if you're willing to "stay close" to the marina. Villa Blanca, where there's a wall and a shallow dive, is between downtown and the marina. Paradise is basically right in front of the marina. Las Palmas, the C-53 wreck and Chankanaab are pretty close to the marina. I'd ask what the shop policy is, and see if they were willing to take a smaller group than 4, or 3, if you were willing to not go far south.