Your Grasshopper account number is actually your Grasshopper phone number. If you have multiple phone numbers on the account, any of them can be used as your account number. Learn how to find your Grasshopper phone number here.
If you are looking for the account number for a different carrier, you can contact them to request a Customer Service Record. If you plan to transfer your phone number out of Grasshopper, a obtaining CSR is the recommended first step.