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 (or RCS)
Turn off "Enable Chat Features" (or turn off RCS)
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.
If this doesn't work, please try clearing the cache for both Google Play Services and Google Play Store:
On your Android phone or tablet, open the Settings app .
Tap Apps See all apps.
Scroll down to "Google Play Services."
Tap Google Play Services.
Tap Storage Clear Cache.
Tap Manage Space Clear all Data.
On your Android phone or tablet, open the Settings app .
Tap Apps & notifications See all apps.
Scroll down and tap Google Play Store .
Tap Storage Clear Cache.
Tap Clear data.
If this doesn't work, try disabling the SIM-card and then enabling it again in
Settings > Connections > SIM-card management.