Could I be getting too many PhD students? [loc-0048]

I'm in a bit of a pickle, although I accept it sounds like an academic "first-world problem": I already have three first year PhD students (Yi, Kai and Xin) that I supervise quite actively. I still have funding for one more international place and I'm currently talking to four additional people who want to do a PhD with me, some of whom are local students so very likely to get funding. The worry is twofold:

1. I'm starting to run out of good topic ideas to help possible students get started on a proposal, and
2. I don't think I'm capable of supervising many more students, at least not in their first year. Once my existing students are further along it might be easier.

I am worried that I might have to start turning good people down — although I don't want to. But I fear that if I accept too many students the outcome will be worse for all of us.