。
She has as many as seven children.
Literal
She [topic-は] seven-people [emphasis-も] children [subject-が] exist.
七人も uses the emphatic も after a counter to express surprise at the number — 'as many as seven!' The も transforms a neutral count into an expression of amazement, suggesting the speaker finds seven children remarkable. 子供がいる (to have children) uses いる for the existence of people. In contemporary Japan, where the average family has fewer than two children, seven is genuinely striking.