关于up to 做复合介词的使用方法的疑惑

老师好，我有个疑惑，up to表示多达....时，修饰的是具体的数目， 那样问题来了， up to做为介词，为何能修饰数词呢？修饰数词不应该是副词的使用方法譬如（nearly about等等）。辛苦老师解答。

这里的up to不是介词，而是副词短语，常可修饰具体数目。《剑桥高阶英语词典》有如下讲解：

up to adverb

used to say that something is less than or equal to but not more than a stated value, number or level:
Up to two hundred people were on board the ship.
We can teach dancers up to intermediate level here.