There are several pools that the dolphins are located in; and Discovery Cove its self has around 30 dolphins, not 12. And I think very rarely are all 3 pools doing DIPS at the same time, it's usually staggered, so that the animals are getting a break.
But as for your swim, you will usually get 1-2 dolphins and will be in a group no larger than 8. But there will be multiple DIP's going on in the pool at once (the way the pp explained).