Message notifications
Why showing message number when there are no messages unread messages
iPhone 13 mini
Hello Henrypm!
If you see the number, there are definitely unread messages somewhere; perhaps deep down into the conversation, but they are there. Try to search for them, or, if you don’t mind, you can delete the conversation, and that will also fix the issue.
You can also try to Force restart iPhone - Apple Support, to see if that will take care of the problem.
I hope this is helpful. Good Luck!