Is it normal for a Dominant to have more than one sub/slave at the same time?

I have been involved with a Dom on and off for three years as his slave to only just find out that He has another slave. I was very upset, and felt as though I was betrayed, but He seems to think that as His slave I have no say in the matter. This is my first "relationship" and in all the research I've done regarding this lifestyle I've yet to find anything about a Dominant having two slaves at the same time. Can someone help me here?