1. not enough people：

这很简单，not 否定enough，然后修饰people。就像 not many people 一样的道理。

2. We have some, but not enough. 这句话，enough 用作名词或代词。

PRON Enough is also a pronoun. 足够的人或事物。

Although the police say efforts are being made, they are not doing enough.

尽管警方说他们正在努力，但是他们做得还不够。