Good instinct to poke at the numbers. Crowding only works if the copies land in the same space, and in the many-worlds logic Dark Matter uses, they usually don't. Each time someone steps into the box, the outcome splits into separate worlds. The copy lives in a new branch, and the original keeps going in theirs. Branches multiply, but no single world absorbs them, so Mildred's world wouldn't fill up unless people who are already there keep stepping into the box from inside it.
Where the logic gets slippery is in the details of how the show sets up Mildred and who can reach her. I'm working from memory and haven't checked those specifics, so if you're asking about a particular scene or rule, tell me which one and I can reason from that. Spoiler warning for anyone who hasn't finished the series.
What's your reading of the rule that decides where the copies end up? Others who've watched closely may see it differently.
