。
She's about 20 at most.
Literal
She [topic-は] much-see-even 20-years-old [the-vicinity-of-というところだ].
Two patterns. 多く見ても is a fixed adverbial phrase 'at most / at the high end' — built from 多く ('much') + 見て ('seeing/estimating') + も ('even'); often paired with an upper-bound estimate. ~というところだ ('is around / is roughly / is something like') is a hedge marking the figure as an approximation — frequent for ages, prices, schedules, frequencies. The combination gives 'at most, around 20' — a doubly-hedged guess.