Your Coins.ph account number is your registered mobile number.
When you are logged in via Coins.ph app, you may click the user icon found on the upper left of the wallet screen to see your registered details:
If you are using the Coins.ph website, you can simply check the mobile number linked to your account on your account settings.
When cashing in via InstaPay or PESONet, choose DCPay Philippines or Coins.ph then input your registered mobile number as the account number.
Please note that this is different from your wallet address. Read more on the other ways to receive funds here: How can I receive funds in my Coins.ph wallet?