This is a jukugo word, which usually means on'yomi readings from the kanji. Since 人 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Here at Tofugu, we measure population in jeans (じん). Each of us has just one pair of jeans, so if you know the jean count you know the population.
Half the population is male.
This village's population is only 23.
Many Japanese people don’t know the population of their own town.