How do you know a person dies every twelve seconds?

This statement is not exactly true. Assuming the number is correct, it is an average.

It was no doubt found by dividing the number of seconds in a year by the number of people who die in a year. Death records are kept, so the number of people who die each year can be found with reasonable accuracy. Of course, as the population goes up, this number goes up and this ratio would go down.