The number of people which lost homes reached as many as 2500.这个句子是否正
the number of意为“……的数目”；a number of 意为“许多”（＝many）、“若干”.例如：
I want to know the number of the motor-vehicles.
我想了解机动车的数目.
The number of books missing from the library is large.
图书馆丢失的图书数量是很大的.
I have a number of reasons.
我有许多理由.
A number of students were absent yesterday.
昨天有一些学生缺席.
你所问句子的主语（先行词）不是people,而是the number(数量),是指“数目”（数目不是人）这一概念,所以不能用关系词who.