The Department of Homeland Security said almost 530,000 people overstayed in fiscal 2015, about 200,000 more than were apprehended at the border that same year.
https://www.bloomberg.com/graphics/2017-trump-mexico-wall/how-many-people-currently-cross/
That's 5x's the number that make it thru the boarder (assuming 2/3 caught and 1/3 make it thru)