As many as five million voters cast their ballots in this election.

many 的意思是"许多"，表示数量多。 as 在这里用作副词，表示"同样地"或"达到某种程度"。 as many as 是一个固定搭配，表示"多达"，强调数量之多，通常用于数字前表示惊讶或强调。