the number of people each year rises one 中为什么rise后加"s"

the number of people each year rises one 中为什么rise后加"s"
The number of the people that/who come to visit the city each year rises one million.
还有一个 The school in which he once studied is very famous.
The school (that/which) he once studied in is very famous.分别仔细翻译一下