As per Notification No 12/2017 - Central Tax dated 28-Jun-2017, the number of digits of HSN Code to be mentioned on Tax Invoice is described. It depends on the preceding Financial Year turnover of the entity.
1. If < 1.5 crore then NIL Digits of HSN Code
2. If > 1.5 Crore but < 5 crore then TWO Digits of HSN Code