By my count, there will be up to 6 regular side missions revealed as options for the rebel team during the course of a core set campaign: two right away, then one additional replacement drawn as an option after each of the first four side missions. A replacement doesn't need to be drawn after playing the fifth side mission because it would never be played anyway.
If the Imperial player puts Agenda side missions into play, it seems to me that they would only increase the number of drawn side missions if the rebels opt not to play them -- then the Agenda mission would disappear (granting the reward to the Imperial) and they'd draw a replacement for the regular side mission that they chose to play instead, just as if the Agenda mission had never been there. The core set includes 3 Agenda side missions and 2 Agenda forced missions, but the forced missions would have no impact on the number of side missions that would come up.
So by my count, the total number of side missions that could ever be drawn and revealed during a core campaign should be 9, and that's only if the Imperial buys all three core set Agenda side missions, and the Rebels decline to play any of them, which is pretty unlikely.
Does this math sound right to you all? Or am I missing some way that even more side missions could be drawn during a single campaign playthrough?