That 36,000 number gets a lot of press these days, but where does it come from?
There's a really nice explanation of it on Effect Measure
Net: there's no straightforward way to do a count, the 36,000 number is just an estimate (based on multiple ways of estimating), and in any event that number varies a lot from year to year. There's more, but I will refer you to the link above.
Interestingly, one of the main methods is Serfling's method. This bears a striking similarity to the measures used to assess the success of sales promotions.