How many years can I go back to do my income taxes

[hfirststar]

Say I didn't file taxes in 2012 till now. How many years can I go back

[Critter]

If you have a refund coming to you then 2014 is the earliest year you can still file and get the return.   If you owe taxes you are required to file all missing returns. 

TurboTax Online only supports the current 2016 tax year and the prior 3 years (2015, 2014, and 2013).   The latter 3 have to be prepared in desktop software (CD/download). Each tax year requires a different year software; i.e., 2015 can only prepare 2015 return; 2014 software can only prepare 2014 return, etc. Prior-year returns cannot be efiled; they must be printed and mailed.

NOTE:  If you prepare returns for multiple years, do NOT mail them in the same envelope. Mail them separately.

You can see prices for those 3 prior years at the link below if ordered directly from TurboTax, but you may also be able to find them at a discount at reputable retailers like Amazon:
https://turbotax.intuit.com/personal-taxes/past-years-products.jsp