The number of stars visible to the naked eye is countless.
Literal
Eye [to-に] visible stars [poss-の] number [topic-は] countless [copula-です].
目に見える ('visible to the eye') is a relative clause modifying 星の数. 無数 ('countless, innumerable') is Sino-Japanese: 無 ('without') + 数 ('number'). A factual, somewhat poetic observation.