As many as 6,000 people may have been infected with the disease.

as 在这里是副词，表示"达到某种程度"或"同样地"。 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"。 as many as 这个短语用来强调数量之大，表示"多达"或"有...之多"的意思。