在（定语）从句中,most of whom修饰主句的主语或表语（名词/代词+of+which/whom）
修饰限定表语
There are 300 desks in the hall, most of which are new.
大厅里有300把桌子,大多数（桌子）都（是）新的.
相当于：
There are 300 desks in the hall and most of them are new.
修饰限定主语
Millions species die yearly, most of which are unknown to us.
明年数以百万计物种消失,而其中绝大部分对我们来说都是未知的.
相当于：
Millions species most of which are unknown to us die yearly.
或Millions species die yearly and most of these species are unknown to us.
供参