He made five mistakes in as many lines.

句中 many 的意思对应前面的数字 five，原句相当于：

He made five mistakes in as many lines.

= He made five mistakes in the same number of lines.

= He made five mistakes in five lines.

他在五行里就出了五个错误。

类似地：

They visited five countries in as many days. 五天内他们访问了五个国家。

He had three accidents in as many months. 他在三个月里出了三次事故。