notafew与quiteafew是反义词吗？

不是的。not a few 的意思是“不少”“相当多”，quite a few的意思也是“相当多”，也就是两者不是一对反义词，而几乎是一对同义词。如：Not a few of the members were absent. 不少会员未出席。He has made quite a few friends. 他交了不少朋友。Quite a few of us are worried. 我们不少人很着急。