London has a population of ten million.

因为 a population of 是固定搭配,表示人口是多少
millions of 解释为“数以百万”的.
但是,如果具体地说出二百万、五百万等,则 million 后没有 s ,也不用 of:
例如：two million smokers.
高考题
1. __ people in the world are sending information by E-mail every day. (2001 上海)
A. Several million B. Many millions C. Several millions D. Many million
(A.)
2. Shortly after the accident, two __ police were sent to the spot to keep order. (MET. 1992)
A. dozen of B. dozens C. dozen D. dozens of
(C.)

a population 怎么会是单数呢? a city, a country
ten million 规定是用单数, 用复数时是 "millions of", "thousands of" 等等