Final answer:
A standing wave that is two wavelengths long will have four nodes, not including the endpoints, because each wavelength has two internal nodes.
Step-by-step explanation:
There are four nodes not including the endpoints in a standing wave that is two wavelengths long. Each wavelength in a standing wave typically has two nodes, including the endpoints. Since the question specifies not to include the endpoints, for two wavelengths (which would normally have four nodes including endpoints at each end), there would be two internal nodes per wavelength. Thus, we have a total of four nodes when not counting the endpoints.