GSTR 9C return

Every registered person whose aggregate turnover during a financial year exceeds two crore rupees shall get his accounts audited as specified under sub-section (5) of section 35 of the CGST Act, and shall furnish a copy of the audited annual accounts and a reconciliation statement, duly certified, in FORM GSTR-9C.

This Limit of Two crore is increased to five crores for F.Y 2018-19.

So, if your Turnover is more than this limit than, GSTR 9C is mandatory.
If turnover less than this limit, than its optional, you can file voluntary, if you want.