The challenge says Available parts is 18 ramps.
The solution shows 19, which is the correct number required
The 18 ramps available are in addition to the 3 ramps, 1 crossover and 1 bit already placed on the board. As you’ve noticed, that gives you 2 more ramps than needed.
Compare with Challenge 1, which has 6 ramps already on the board and lists 4 ramps as available.
Thank you for the explanation.
That satisfies my comment.