How many previous years’ ITRs can be filed after due date?

Hi Sree, Hope you are doing well. One of my employees has never filed ITR since 2012. He started his career in 2012. From 2012 to 2014, there was no tax deducted from him as his salary was less than the tax limits. In 2014, he changed the company and started to pay a huge tax from 2014 to till date. And he has all the Form 16 part A & Part B from all of his employers since 2012. But he was not aware of the ITR and never filed any ITR though he is a salaried employee and paid tax on time without any dues. Now, my query here is, for how many previous year's Income tax returns he can file? (I believe it is the past 2 years. But is there any way to file for all the years since 2012). Will there be any penalty, if he files ITR now? What would be the impact as he failed/forgot to file ITR since 2012? Thank you.