Thanks for asking the Apple Support Communities for help with addressing Messages. What you are seeing is your previous recipients list.
You can clear the previous recipients list using the following steps.
1. Create a new message and begin to type the name or number of the contact you wish to remove.
2. When it appears in the list, tap the circled “i” button on the right to bring up the contact info window.
3. Here, you’ll find a red button labeled Remove from Recents. As its name suggests, tap it to delete that unwanted number once and for all.
Once the information has been removed, you’ll be returned to the New Message window. This time, you’ll notice that the unwanted number is gone, and that only the correct information from your Contacts app is displayed. Thanks and have a great evening.