In a statement of purpose for a PhD in computer science in the US is it better to express interest in a single subdiscipline or multiple? It seems that if I were to just express interest in a single subdiscipline (computer networking for example), and none of the networking professors are taking on students, I would not be considered based on any other merits my application may have. But if I were to express interest in several subdisciplines it may be difficult to create a structured statement of purpose.
A PhD is about expertise in a narrow field. Showing too wide spread interests is counterproductive, you should have a rather clear idea what your principal interest is. It is OK to have wider interests, but you need a focus.

Do your homework beforehand, select prospective advisors and contact them for openings.