You need to check your ego at the door with this one because it will not be anywhere near your total number of followers. But there has never been a lot of substance to that stat anyway.
By targeting just those people who have engaged with your content, you are focusing in on the active followers.
I recently did a small boost (just $1) of a client's post and targeted their content engagement audience. The boost reached an extra 3,000 people and brought in an additional 468 engagements at a cost of less than 1 cent per engagement.