Disney buses hold 72 people.
Disney will, on occasion, provide dedicated service of three bus loads or more,to very large groups, like BTGs, that typically travel together.
Doing this, is both a service to those guests and it also avoids bogging down the system and creating a major inconvenience to other guests who would be unlucky enough to be traveling at the same time, to the same destination.
So to answer the OPs question; if you will have a couple of bus loads in your group, I would contact Disney and explain the situation. Keep in mind that they will never keep these buses out of service for very long so everything is done on a very strict schedule.