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 settings
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.