You can't stay for more than 6 years on an H1B unless your employer files a PERM application, has that approved, and then you/your employer files an I-140, and that is also approved. In other words, if your employer never filed a PERM, or never had one approved, you have to leave after a maximum of 6 years.
Green card applications for Indians (and to a lesser extent Chinese and Mexicans) are backed up for decades. So you're looking at people who filed PERMs in the last decade and are still waiting for green cards. They're allowed to stay on the H1B, which is why we have 800k people on H1Bs today. But these people are not working for Infosys, they're working for Google and Microsoft.
The 86,000 number contains 20,000 visas allocated to people who have graduate degrees from US universities. I don't think any of these folks are working for Indian consulting firms. So that leaves about 66,000 visas and 40,000 of those, at most, go the consulting firms. I did make an error in my math, which is not accounting for the fact you can stay for 6 years on this visa. Anecdotally, I know that most people of these people leave after about 2-3 years, when their "project" ends. But anyway, let's assume the worst case. You may have 240,000 people in the US on an H1B and working for an Indian tech consulting firm. That is still a small fraction of 3.6 million.