Does a UML Component diagram describe just a single component?
By way of analogy, a SysML IBD diagram describes a single instance of a block type. (Where the block instance name is given by the diagram's model element name.)
Or does the Component diagram frame (usually) contain multiple components?
Maybe every top-level box inside the Component diagram frame is a component. But to say that every top-level box in a Component diagram frame does not seem correct. A UML component (element) has a specific UML definition.