Delete old iMessages
Refer: https://www.igeeksblog.com/how-to-clear-imessages-chat-history-on-mac-os-x/
In Finder, "Go to Folder:"
~/Library/Messages
Delete the following:
chat.db
chat.db-shm
chag-.db-wal
Macbook Unread Message Wrong
Close your iMessages and then open a terminal and do the following:
$ killall Dock
Refer: https://emmanuelbernard.com/blog/2015/02/20/unstuck-unread-count-apple-messages-app/