This is governed by SEBI (Issue of Capital and Disclosure Requirements) ICDR Regulations 2009 [last amended on March 6, 2017.
For ease of understanding, I am posting a gist from an article published by Livemint1 which suggests it is currently b/w 75-90% subject to certain conditions as below:
"Companies with a post-issue capital of less than Rs.4,000 crore will have to sell a 25% stake or stocks worth Rs.400 crore, whichever is lower, to the public in an initial share sale, while a company with a post-issue capital of more than Rs.4,000 crore is allowed to offload a minimum of 10% in the IPO. However, if an IPO of Rs.400 crore size is not equivalent to 25% of its post-issue capital, the minimum public shareholding of 25% has to be achieved within three years of listing."
