I don't think the number of the people ( )this happens ( )ve

这句话的意思是：我认为这件事涉及到的人数不多.
happen to 的to 是介词,后面的引导词应该是that而不是 who.
this happens to 修饰 the number of the people.
所以你可以这样理i don't think the number of the people (that this happens to) is very large.