This city has 5.9 million people.
= There are 5.9 million people in this city.
= The population of this city is 5.9 million.
= The number of the people in this city is 5.9 million.
= The city has a population of 5.9 million.
建议楼主采用最后一种变化,因为它是最地道,最符合英语表达习惯的说法.