161957

We can’t use all of the numbers because:
For the number line going from 0 to 20, you cannot use zero as you wouldn’t be adding or taking away anything so you would stay on the same number.

For the number line going from 1 to 20, you cannot use all the numbers because there will always be one number left over. Each new calculation uses two new numbers apart from the first calculation which uses three so if you keep adding two to three you will always be on odd numbers and 20 is an even number, not odd.

Good ways to win are to try and use up the numbers 1,2,3,4 possibly 5 and 6 because you can get to many different numbers by adding those whereas with the larger numbers you cannot get to so many because you can only go up to the number 20.