The book uses “the sun” as an example. Within its historical illustrative framework, observing one sun does not mean that the generic concept automatically rules out a second possible instance.
If “sun” is considered as a kind of luminous celestial body, nothing in the general concept alone creates a contradiction in conceiving several such bodies, even when only one is observed in a given setting.
Criterion: one actual instance, others conceptually possible. This is a logical illustration, not a claim about the number of stars in modern astronomy.