。
The number of women who become mothers is declining.
Literal
Mother [become-となる] women [gen-の] number [subject-が] few.
となる is the formal/written version of になる ('become'), with an emphasis on the result as a fixed or final state — often used in official, news-like registers. 母となる女性 is a relative clause modifying 女性 ('women who become mothers'). The whole sentence reads like a headline or a demographic observation.