Demat transaction charge (DTC) is charges only on delivery sell side and not while buying. You will be charged Rs.15 plus GST (Goods & service Tax) for each scrip, per day on date of selling whenever you sell a scrip on a delivery basis.
Ex: You are selling delivery shares of SBI 100 qty on Monday at 10am, 50 qty at 11am, 10 qty at 2pm. You will be charged Rs.15+ GST of DTC for SBI scrip. On the same day if you sell any other stock with any quantity, you will be charged again Rs.15 DTC. Totally, you sold 2 stocks(scrips) on a given day, hence you are required to pay Rs.30 for that day.