From a Dom's point of view, I find it takes a lot of thought, feeling, planning, caring, emotion, and intellectual power to fully exercise the dominant side of the human experience. Creativity is a must, since you are the initiator of the scenes. Next to lack of communication, boredom is the biggest killer of a relationship. It takes dedication on both sides to make the relationship work.
I have had two subs active at the same time, but never more than that. Trying to devote the attention required to a stable of several subs I feel is not especially fair to either party, and frankly I am not sure I am totally capable of doing it to my satisfaction, or that of the subs.
When I am in a relationship with a sub, I want to be satisfied from her, but also to satisfy her and ALL her appetites. Dissipating my focus weakens my intensity. Call me a traditionalist, but I feel the fewer the better.