As per Revised Model GST, Chapter VI-Sec-23(2) of Registration, every person who is liable to to be registered under schedule V of this act, has to apply for registration "business vertical wise" within the state, but no need for product wise registration.
As per your queryt being a retail shopkeeper of both un-stitched cloth and readymade clothes no need to have separate GST number for both categories.Thery may be difference HSN code for this two products and rate may varies. Product wise rate based on HSN is yet to be finalised by GST council.
Meanwhile conculde the GST migration process for your existing VAT number to GST on GSTN.