We are using 10 digit format for mobile phone numbers in India, means a billion numbers maximum. For a population of 1.2 billion+ people, this 10 digit format is logically, practically is not enough.
As of now, a person can buy 9 mobile numbers legally in India. Few years back, there is no restriction for that, so some people bought hundreds of sim cards and retailers also misused the system.
An average Indian owns at least two mobile numbers, one for professional/business use and the other one for personal use. Thanks to the dual sim enabled phones.
Every month, there are 2 million new sim cards sold by the network operators.
For now, the network operators recycling (reuse) the Inactive mobile numbers to solve the number outage issue. But sooner or later, 11 digit or 12 digit mobile numbers will be circulated.
But increasing digits is not a child's play, it will collide with international numbering standards. DoT (Department of Telecommunications) only knows the future of numbers.