Suppose imagine you have 2 cards and 2 envelopes: 2 are blue and 2 are yellow. Now, in how many possible ways can you arrange them in a pair? Blue card-yellow card, blue card-blue envelope, blue-yellow envelope, blue envelope-blue card; 4 ways.
This answer could also be predicted by using "Permutations and Combinations", which is simply math combined with logic. Simply put, the equation is as follows: 2 (number of cards) x 2 (number of envelopes) = 4.

The increase in population 
Although it does not appear to be directly linked, the same logic can be applied to the number of digits in mobile phone numbers. This depends solely on the number of mobile phone users, as the number of available mobile numbers should obviously exceed the number of users.
India's current population is 1,367,938,326, according to the latest United Nations estimate calculated on 2019. Considering that the population is expected to increase exponentially to a whopping 1.7 billion by 2060, the amount available mobile numbers must be greater than this. estimate.

All about Numbers
Going back to Permutations and Combinations, how many single-digit numbers do we know? 10. So if phone numbers had 2 digits, how many different number combinations would be available? 10 x 10 = 100. 
In the same way, if phone numbers were 9 digits long, this would only satisfy 100 crore (1,000 million) subscribers. Hence the 10-digit mobile number. 
This gives us 10 billion (1000 crore) of different combinations and, in terms of the world's population, it is highly unlikely that it will exceed this capacity.

What do the digits of a mobile phone number represent? 
What does an Indian mobile number represent? Well, it's not just a group of randomly aligned numbers. All mobile phone numbers have 3 things in common: a 2-digit Access Code (AC), a 3-digit Provider Code (PC), and a 5-digit Subscriber Code (SC). Let's consider the following mobile phone number:

985-677-5683 

AC = 98 

PC = 567 

SC = 75683

Why India have 91 as country code

COUNTRY dialling codes were agreed in the early 1960s by the International Telecoms Union (a club of each country's network providers). In the days of mechanical telephone exchanges, it was convenient to split the world into broad areas: 1 for USA, 2 for Africa, 3 and 4 for Europe, etc. These were then further divided according to the capacity of each network, 33 for France, 44 for the UK, etc, and further still for smaller countries: 353 for Ireland, 354 for Iceland. The method ensured that no more than 11 digits were dialled, including this code. This was needed by the exchanges, but also avoided confusing the public with unnecessarily large numbers of digits. Modern electronic exchanges do not have such limitations - as shown by the recent changes to codes in the UK and France, for example. Politics may also have played a part in the allocation, e.g. 1 for USA so 7 for the USSR. The UK and France were the main players in the ITU, so they took the "nicer" codes 44 and 33. This country code can be varied from single digit to three digit. India has two digit country code +91.

An article by Munna Suprathik