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.
Below are steps to check your Demat account number with Upstox.
Step 1: log into your Upstox account from your mobile and go to the 'Profile' Section.
Step 2: Click on “My Account”
Step 3: Click on “3 lines”
Step 4: Click on “Basic Information”
Step 5: Check your Demat account number with Upstox.
Step 6: This is how you can check your demat account number in the old Upstox app.