so such 加名词 什么时候只用so 是表示数量的时候吗？

such 是形容词，修饰名词或名词词组，so是副词，只能修饰形容词或副
词。so 还可与表示数量的形容词many，few，much， little连用，形成固定搭配。

so + adj. such + a(n) + n.
so + adj. + a(n) + n. such + n. (pl.)
so + adj. + n. (pl.) such +n. (pl.)
so + adj. + n. [不可数] such +n. [不可数]

so foolish such a fool
so nice a flower such a nice flower
so many/ few flowers such nice flowers
so much/little money. such rapid progress
so many people such a lot of people

so many 已成固定搭配，a lot of 虽相当于 many，但 a lot of 为名词性的，只能用such搭配。
so…that与such…that之间的转换既为 so与such之间的转换。

当名词前面的修饰语为表示“多、少”等意义的many、much、few、little等形容词时，只能用so.

so many books
so little food

当名词前面的修饰语为表示“多、少”等意义的many、much、few、little等形容词时，只能用so.

so many books
so little food

结构不一样
so beautiful a girl...
such a beautiful girl....

结构不一样
so beautiful a girl...
such a beautiful girl....