Damn, this thread went wild pretty quickly.
Just kidding, I appreciate that everyone stay civilized in the discussion, regardless how implausible and illogical the topics goes. That being said, I'm not really a fan of super big numbers for groups, unless if you have a really good reason for them. This is a basic worldbuilding learning here; before you create a country, group or alike, you need to figure out what is realistically achievable, and whether something is truly achievable.
I won't ever say "no you can't have big numbers" but please be reasonable. This goes EVEN to M.E.G themselves (sorry all mods behind the M.E.G but we gotta talk about those big numbers and the theories behind them) because it's matter of plausibility. Backrooms allows unrealistic nonsense, but we still gotta figure out whether something is truly possible or not. M.E.G built by superhuman figure such as the First, and lord knows what else that they have there, so they have their excuse.
"But Crim, the Voidseeker cult has millions of members!"
They're spread across the Backrooms, and I would argue that their existence is tied to anomalies across the Backrooms. A single anomaly could have interact with hundreds to thousands of peoples in a single level, in cumulative span of several years. Millions is totally possible, considering the extreme lore behind the group. Beside, they're tied to powerful entities and powerful groups. There are reasons why they're that big.
So if you wanna write crazy things, better make sure that they're make sense.