We only call it incest when the nearest common ancestor is a parent or (depending on the jurisdiction) a grandparent.

If your nearest common ancestor is a great-great-great-great-grandparent, that's not incest (it would most likely not even be known at the time of marriage), but it still leads to large amounts of duplication in your family tree. Every family tree will have many instances of nth-cousin marriages simply because people tend to marry people who live close to them.

It's very hard to calculate what the actual number would be because it depends a lot on where your ancestors lived and how much they migrated over time.