。
She's 18 at the most.
Literal
She [topic-は] much-see-even 18-years-old probably.
多く見ても is a fixed adverbial 'at most / at the high end,' built from 多く ('much') + 見て ('seeing/estimating') + も ('even'). Pairing it with the conjectural だろう ('probably') makes the upper-bound estimate the speaker's own conjecture: 'I'd say she's 18 at most.' This kind of double-hedged guess — 'at most' + 'probably' — is typical of speculative talk in Japanese.