Using Factions as a trigger for the end game

So, going to run a game of Scum and Villainy this weekend, and we’re using the premise of the famous Traveller campaign Pirates of Drinax (the long and short, there are “Aslan” raiders who will invade Procyon, and the governor Malklaith is getting the PCs to gain him allies, artifacts and attention, so he can both fend of the invasion AND proclaim himself a King within the Hegemony).

Anyway, in the book, it is noted that basically once a crew reaches +3 with a faction, a sort of end game series of missions is triggered to “change the sector”. Now, based on the premise above, I want to have it so that there are an array of Factions at +3 and +2 (and I imagine, factions at -2 or -3 that will side with the Aslan when they invade). I like this idea and want to run with it.

My question is - how many factions (either positive, so the Governor feels confident he can win, or negative, so the Aslan feel confident they can win) to trigger the invasion? Basically, I want to have when it happens depend on the PCs. Just not sure what a good number of factions, and at what point values, are good. Hell, maybe it should be based on Tier (i.e. they get at least 15 points of Tiers in one column or the other before things trigger, so like, it could be three tier 5 groups or five tier 3s or whatever).

Keep in mind, we’ll play maybe 12 sessions total, but in each session we’ll get done probably 2 “jobs” (we played Blades before for a campaign, and were at the point we could do 2, and once even 3, jobs in a session of play). Thoughts?