Your Grasshopper Connect account number is actually your Grasshopper Connect phone number. To find your phone number, tap the Settings button in the top left of the toolbar. You'll see it listed under your name. 

Note: 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.