The DEMAT Account Number is a 16-digit number. Eg: 12081800 00123456. The first 8-digits of this number are the DP ID and the last 8-digits are the Client ID. You'll also get your DP ID when your account is opened with Upstox.
You can check your Demat Account Number and correspondingly DP ID by visiting the profile section from the app/web.
1) Go to the profile section -->
2) And scroll down to view your Demat account details