The rules for All Main Missions (Multiplayer) only state that the run has to have more than one person. Is there an upper limit of how many players are permitted within a single run?

I would say no, though if the other mods have a different opinion I'm all ears.

We had a discussion about it once, at least me and a few of the runners. If I remember, we figured that after some many players, you really lose whatever benefit you might gain. Imagine trying to not only get gear for but coordinate 32 players or more! In the end though, as Mavik said, If others have a problem with this, we can talk about it and potential put a limit on the players. I may be biased, I want to see a run of 50 or even 70 players, just something crazy.