Six times as many people as the population of Japan
「日本の人口の6倍もの人々が」が
'Six times as many people as the population of Japan'
「日本の人口の6倍もの人々が水不足に苦しんでいます。」を訳すれば
'Six times as many people as the population of Japan do not have enough clean water.'
水不足が 'not enough water' ですが、飲用水とのことですから 'not have enough clean water' と言わなければなりません。
The total number of people around the world who suffer from water shortage is about six times the population of Japan.
この場合、次のような言い方ができますよ。
ーThe total number of people around the world who suffer from water shortage is about six times the population of Japan.
「世界中で水不足に苦しむ人たちの数は、日本の人口の約6倍です」
water shortage「水不足」
six times the population of Japan「日本の人口の6倍」
ご参考まで!