In Interstellar the population of Earth has decreased drastically. There are ongoing efforts to simultaneous repopulate the Earth and get everybody away from Earth to somewhere more hospitable.
But how many people were actually living on Earth at the start of the movie?
I found two numbers in the movie. Early in the movie Donald speaks about his childhood and says:
But six billion people...
...just try to imagine that.
Which seems to imply that compared to the current population of Earth six billion people is an unimaginably large number. (I am guessing Donald uses the redefined billion of 10⁹ and not the original definition of the word meaning one million squared.)
Later in the movie during the discussion about which planet to visit first Cooper says:
I am thinking about my family and millions of other families, okay?
Which implies at least 2 million families. And given the effort to repopulate the Earth. I would assume an average family consists of at least 5 persons.
From that I get that the population is at least 10 million people and not even close to 6 billion people. Is there any other information in the movie or other official sources which can narrow the population size down further than this?