In case of interstate supply of service, registration is not mandatory having turnover upto 20/10 lakh. however is case of in case of interstate supply of goods registration is compulsory.
Registration become compulsary in the year in which turnover is more than prescribed limit. suppose your total turnover exceeds 20 lakh on 14 september 2020 you became liable to be registered on said date and application for registration shall be made within 30 days from the said date.