The number of books sold this month has reached 2,000.
这里是不是 the number of books (which was) sold this month has reached 2,000?
the number of books was sold this month. 这句话对不对呢?
你的句子中的 sold 是过去分词作后置定语,相当于定语从句 which has been sold。
但不能像说的那样将 sold 改成 were sold,你没有注意到后面的谓语动词 has reached 吧。