I don't have a lot of experience with poly, but when I think about it, I can think of the following situation (not the only situation possible of course):
- Two live-in primaries.
- one secondary (with their own primary(ies), possibly married to someone involved with one of my primaries, or something like that) that I don't live with and don't have serious life plans revolving around but who is a regular part of my life.
- a bunch of tertiaries, either friends with benefits or people I just can't see very often. The exact number could vary, but since I'm likely to be able to see them only something like once or twice a year at conventions or the like, the only important thing would be that I can see them at different conventions, or at a con that as more days than I have tertiaries there. I'd want to have one night for each of them at least.
Each of these people could have other partners, and ideally they probably would (or have the potential to at some point), as I think a mono/poly relationship would be more challenging.
Of course, I'll take what actually happens. I have no clue what that will be, and it's unlikely to match this scenario. However, this is the kind of thing I think I would be able to deal with. Three primaries seems too much to me, and (possibly weirdly) so does two secondaries. I guess it seems easier for me to have two primaries because I'd be living with them and therefore not "sharing" my time between them as much, or a bunch of tertiaries because I'd see them so little that it wouldn't affect our relationship much if I have like five of them or something, but a secondary is at that mid-point where I think I'd deal better with only one of them.