How many subs can a master have?
I think that's like asking how many children can a person raise properly. Some people have enough energy and resources for one; some can handle seven. It also depends upon the dynamics of all people involved. My little one needs so much attention that I don't have too much to spare to devote to other subs (I've had temporary subs and an online sub, but they all come second to him and I've made that clear from day one), but I love that; I love how he needs me all the time, every minute. That would drive some other doms crazy. So it really all depends upon your individual personalities.

My advice is this: Every now and then, sit yourself down and ask "Am I happy with things how they are?" and then also ask her the same question. If the answer is yes for you both, don't mess with it. Don't change stuff just because you think you're "supposed" to. If the answer is "no", then talk about the reasons why and then decide what you might want to change to increase your happiness. And since people change, you need to ask yourselves this question on a rolling basis.