The limit of 20 lakhs is for registration and not for liability to pay tax. Liability to pay tax arises when you are liable to registration and supplying taxable goods or services or both.
Once you are registered and supplying taxable goods or services or both, you have to charge tax in invoice regardless of your turnover.
If your turnover is below 20 lakhs you are exempted from registration but as soon as you take the registration you have to charge tax in invoice.
chatCommentsSuppose i have turnover say 15 lakhs as on 31st December 2017. I got registered (New) from 1st January 2018. As per your reply i have to collect GST on every supply irrespective of turnover (as exemption is only for registration). Now my query is whether i have to pay GST out of my pocket for the first 15 lakhs (supplies made before registration) as i would have not collected GST on such supply. - Charan RajYou do not have to pay on 15 lakhs. - GST Payer