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。