16, 17, 18, 19 are the "ABXY" buttons (starting from the east and going clockwise). 1, 2, 3 and 4 are the red buttons (left to right). There's no Button 5 (or 6) as I think those are actually the shift paddles.
On the wheel, the red buttons are (from left to right, row by row)
Button 8 | Button 7
Button 21 | Button 20
Button 23 | Button 22
So as you can see, a convenient gap of two. my 50p is on the paddles.
Just rebind in the escape menu options! You can bind to the wheel and keyboard keys simultaneously...