The survey found as many as 70% of respondents were dissatisfied with the service.

as 在这里是副词，表示"达到...的程度"或"像...一样"。 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，合起来表示"多达..."或"和...一样多"的意思。as many as 70%表示"多达70%"或"高达70%"。