As many as 90% of them went to the cinema on a regular basis in 1990.

as 在这里是副词，表示"达到...的程度"。 many 是形容词，表示"许多的、大量的"。 as...as 是一个固定搭配结构，第一个as是副词，第二个as是连词，表示"和...一样多"或"多达..."的意思。 as many as 90% 表示"多达90%"，强调数量之多。