How many of them 的意思是“他们当中有多少人”，这里的 of 表示整体中的部分，不可省略，否则就错了。同样，下面句子中的 of 也不能省略：
How many of them volunteered? 他们当中有多少人主动要参加？
How many of them will there be altogether? 他们总共会有多少人？
How many of them did you invite? 他们当中你邀请了多少人？
How many of them are taking the test? 他们当中有多少人要参加考试？
How many of them finished the test in the allotted time? 他们当中有多少人在指定时间内完成了测试？
