can i keep the stocks which i bought as cnc but i have to keep it long until the price reaches high
Yes, you can. CNC orders deliver shares in your demat. Therefore you can carry them as long as you want.
you can read about product types here:
CNC - Buy today and hold until anytime OR buy today and sell today (auto considered as MIS).
MIS - Buy today, sell today (compulsory).
Short sell is only possible on MIS and not CNC.