曼哈顿语法提问！a number of和many的区别？

erindong

第四章的练习中有一句话的正确答案

Jim is trying to reduce the amount of soda that he frinks at last night's party, however, his resolve to drink less soda was sorely tested, and he found himself quaffing a number of sodas.

原句中 a number of 是many，答案解释说a number of is an appropriate modifier for countable things 但是many不是也可以修饰可数名词么
请问这两个词／词组有什么用法上的区别？

玛妮

因为划线部分没划of