I went to set a supply line to a settlement and it was grayed out. That settlement is max settlers but I thought the supply line guy stayed a resident of the settlement he was sent from??

Nope...the receiving end needs a space as well.

But, it doesn't count against beds/food/water on the receiving end, as far as I can tell.

It needs space but is it counted as a settler on the receiving end?

Oh thats handy to know. I haven't had a max population anywhere yet.

Thaaaats weird.