If Google Messages doesn't show the number you're expecting please try the following steps to resolve the issue:
Disable Google Chat features
Open the Messages app
Click the three dots in the top right
Click Chat Features
Turn off "Enable Chat Features"
Turn your phone off
Re-enable Google Chat by following the same steps as step 1 but toggling "Chat Features" to "On"
Send a test message and you should see your number has updated itself.