How many magical girls are there as a rough estimate? I would guess if they're all over the world, at least one per city would make sense, with larger cities such as Tokyo possibly having more? Of course it's up to you though.

However, going with the rule of eliminating the girl with the least amount of charms, that won't work as well for larger population sizes. Eliminating one out of 10,000, for example, isn't all that threatening. XD