A test account was used in the last UE to kill 1 Lillim. At the end of the UE that ranked into the 7k range.
Keep in mind again that it was 1 Lillim & nothing else. Ranked 7k. I'm pretty sure people would universally agree that 1 Lillim can not be considered "active."
From that lil' test, I'd estimate the actual "active" population of participants in the UE to be closer to ~ 3-4k players (ppl that killed more than just a few lillims). I put "active" in quotes because that definition probably differs from player to player. Because 3m PP ranks around top ~600 actually (I use 3m as an example, simply because 3m just completes the individual PP rewards of an UE at least).
Now if you take that 3-4k total, and split it evenly into unions, you'd get about 100-133 possibly "active" unions. Of course mixing actives with inactives, those numbers won't actually hold true in the game.
But still sheds a bit of light about the playerbase here...