most of which 是用来引导定语从句的,前面必须有个先行词.
most of them 是一个分句.
i.e.I have many foreign language books,most of which are world famous works.(我有许多外语书,其中很多是世界名著.)
i.e.I have many foreign friends,and most of them are from USA.(我有很多外国朋友,他们当中的很多人是来自美国.)