I'm just curious at the appearance of 「口」 that makes this word mean "population". Why should it be 「口」 as opposed to any other body part or anything else? Is there a definitive reason or story associated with this word? If 「口」 is used as a counter then why couldn't 「個」 replace it? If I'm not mistaken doesn't 「個」 refer to individuals more so than 「口」?
In old Chinese--where Japan borrowed the term--口 was a counter for people. More specifically, it referred to the number of people needing food. English has this construction too: the number of "mouths" (=people) to feed.
As for 個, it generally refers to individual non-animate items.