I forgot to file 8606 for non deductible IRA in 2017 and 2018 , in order to correct do we need to file amendment 1040X for 17 & 18 or can we file 8606 without amendment..

You do not need to file an amended return in order to file a Form 8606.  Form 8606 can be filed by itself or with a tax return.  The forms are available at IRS.gov.

Thank you for your response David,really appreciated!

Q1) Can i use turbo tax to file prior year 2017,2018 "8606" and file electronically or i need to file through mail only

Q2) Do i also need to pay 50$ penalty/form, or an explanation letter is fine.

You will need to file 2017 and 2018 amended returns that will include forms 8606 in each return.  These will need to be printed and mailed.  There can be a penalty of $50 for not filing Form 8606 on a timely basis, but the penalty can be waived if you can show reasonable cause for not filing. Include an explanation letter along with the rest of the printed paperwork.