You can locate your phone number by going into TexNow and tap the menu button on the top left to show the pull-out menu as seen in the picture below.
Please ensure you've signed in with the same username you used to purchase or activate your device. If you are signed in to the correct account, you should not be seeing any advertisements, and be able to see your data usage as well as the Account option inside the pull out menu. If that's not the case, you should logout and login using the same TextNow account you used to purchase or activate the phone.
If you see a different phone number in the Settings application (under "About Phone"), you should not be alarmed. This phone number is used for internal purposes only, such as identifying the device on the Sprint network. You can safely ignore this phone number.