In the world due to technical and user base reasons the mobile numbers at present varied from 10 to 11 digits .Due to technical reasons 2 countries UK & china moved to 11 digits in mobile numbers .In india all mobile numbers have 10 digits under the government's National Numbering Plan(NNP).
Another reason might be Population of India, which is 1.21 billion So,
The maximum number of 9 digit numbers is 900 million
The maximum number of 10 digit numbers is 9 billion
At present all mobile phone no.s are 10-digits long and the way to split the no.s is defined in the National Numbering plan 2003 as XXXX-NNNNNN where XXXX is the network operator ,NNNNNN is the subscriber numbers