The FAA currently estimates that there will be 30,000 drones licensed to operate in US skies by 2020. It's a misleading estimate.

Why?

It only counts large, professional drones (and even that estimate is low). It doesn't count all of the small/micro drones operating below ~400ft and at slower airspeeds. How many micro-drones will there be by 2020?

Tens of millions (tens of billions if there is warfare or repression driving it -- and given the problems we are facing, there will be) ...

How do we get to that number?

Simple. You print them by the sheet.