When drive-ins were at the height of their popularity in the late 1950s, some 4,000 existed in the United States, but there are fewer than one-quarter as many.
在 1950 年代后期，当免下车进入最流行的时候，美国大约有 4,000 辆，但只有不到四分之一。

as many ：同样数量的
如：twice as many, one-quarter as many，We visited five countries in as many days。