。
She didn't drink the coffee with lots of sugar in it.
Literal
She [topic-は] sugar [subject-の] much went-in coffee [object-を] did-not-drink.
Inside the relative clause modifying コーヒー, the subject of 入った (went in/was put in) is 砂糖 — but the が that would normally mark a relative-clause subject has shifted to の (砂糖の多く入った). This が→の swap is routine within shorter relative clauses. 多く is the adverbial form of 多い ('many, much') — i-adjectives can act as adverbs when their final い becomes く. のまなかった is the past negative of 飲む ('drink'), here written in kana for a softer feel. The whole structure shows how Japanese stacks dense modifiers in front of a noun without any connective word like 'that' or 'which.'