I don’t know how, but all of my text messages that were sent to a certain number have been completely erased or deleted. How do I recover that

If you're using iCloud Syncing for Messages, they are not part of a back up. Once they are deleted, they are gone.

If you're not using iCloud syncing and you have a back up that contains the messages, you can restore your phone from that back up.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Note that restoring your phone from a back up will overwrite everything currently on the phone.

Messaging combines all texts between your phone and another specific number as one thread. If you delete what you think is one message the whole thread is deleted. That is probably what happened. It is possible to delete a single message, but it is a more complicated process than just swiping the message and tapping Delete.