Answer: n-1
Step-by-step explanation:
Let's say we're dealing with a quadratic. The degree is n = 2. It has n-1 = 2-1 = 1 turning point, which is the vertex (either highest or lowest point depending how the parabola opens).
Now consider a cubic with degree n = 3. It has at most n-1 = 3-1 = 2 turning points. It could have less turning points, or none at all.
By definition, a turning point is when the curve goes from increasing to decreasing, or vice versa.