You can find your account number on your Optimum or Suddenlink bill or in your Optimum or Suddenlink online account.
Optimum Customers
Your account number is at the top of each page of your monthly bill or at the top of the page when you sign in to optimum.net/pay-bill/
Suddenlink Customers
Your account number is at the top of each page of your monthly bill or at the top of the page when you sign in to My Account.