Chapter 40 - One-Dimensional Quantum Mechanics - Conceptual Questions - Page 1212: 3

$6$

We can see from the given figure that there are 6 peaks. And excluding the endpoints at the potential walls, we can see 5 nodes; where the number of nodes is given by $n-1$ where $n$ is the quantum number. Thus, $$n-1=5$$ Thus, $$n=\color{red}{\bf 6}$$