How could so many drones establish contact with ATC?
I'm not sure about everywhere but in the U.S. there is an app called "Airmap" Link to webpage. It's built for commercial drone operators but it can also be used by people flying for fun. 

There is a flight plan portion of the app that allows you to file for a flight. You specify time of takeoff, radius you'll fly in, altitude and aircraft. This is then submitted for approval from the FAA.

It takes a matter of seconds and your approved, or not depending on the situation. It also has a map portion that displays the other approved flight plans in the area, if the pilot made his public, so you could even use this to join other's flying at the park if you see a flight pop up or something like that.

This could be a viable answer for India as well, coupled with a mini ADS-B, like CrossRoads mentioned, could prove even better.