It depends on a lot of factors, but the limit of primaries for any situation is eight. I can prove this with with a fairly simple equation...
Actually, I'm pretty terrible at math, and that was all bullshit. Well, except for the "many factors" part; that's probably true.
For me personally, I have no idea. At most, I've only dated two people at the same time, one could probably be called primary and the other secondary in the one situation I was in. With what I have going on now could be two tertiary relationships, or possibly secondary, but none of this is really an accurate description.
One of the reasons I started to identify as polyamorous in the first place is so I would stop trying to quantify everything. It's all friendship to me. Whether it takes on the form of a primary relationship, a "friends with benefits" thing, or even Platonic friendship; it's all the same. The intensity is different from person-to-person, and even varies in each individual relationship. I just enjoy the various connections I make with people, and try not to worry too much about where I want the relationships to go. In my experience, deciding what I want from a relationship before I know what's possible is the best way to assure it never gets to that point.
The magic number for me might be three, or ten, or one... possibly zero. Most of the time for me is is zero, and I'm perfectly happy with it.