。
Many people didn't notice him in disguise.
Literal
Disguised was him [to-に] didn't-notice people [also-も] were-many.
The relative clause 変装していた modifies 彼 — 'him who was in disguise.' 気づく takes に for the thing noticed. 多かった is the past of 多い as a predicate. The も after 人 suggests these oblivious people existed alongside others who did notice.