My charisma is 7 but I have 21 settlers at a couple of places but at others can never o past 16 no matter what I do, is this normal? I can never get above 16 at Sanctuary.

Do you wear any clothes, armor pieces, or have used consumables that add Charisma?

Because the game has bugs and when you are not in the settlement, it can count settleres incorrect. When I leave Spectacle island, it often shows me 15 settlers , but there are really 20 settlers. And I can send there more settlers because of this bug. Also if you temporary use some clothes or chems to boost your charisma, it probably can cause the same effect. If you don`t want more settlers, then you should switch off radio beacons.

This. If it happens that the "settler tick" happens while your charisma is artificially raised (by clothes or chems) the max population is raised to that level.

This way you can even send settlers to settlements that are "full"... or dismiss companions to those places.