The GST Council, in order to reduce the compliance pressure on small businesses, has set a limit of annual turnover of Rs.20 lakhs for registration under GST. Businesses with turnover below this threshold do not require GST registration.
Turnover includes actual sales and sales by way of samples and that needs to be kept in mind while calculating your turnover.
But to answer your question, there are certain exceptions to this rule. Firstly, if the supply of good is inter-state, then GST registration is mandatory irrespective of the value of turnover. Also, this threshold limit is only applicable to manufacturing and not services.
Hope this answers your question!