The function is like a toggle switch, Notify sends an email (to your email address used in my.avast.com/forum) to notify of any new posts in the topic. Once you click that the notify changes to Unnotify, that is used to reverse the decision to notify you about new posts.
For this to work correctly you have to be using a legit/valid email address, which you say it has.
There is no minimum post count for it to work.
I don't use this feature, I choose to use the 'Show new replies to your posts.' So I don't know if there are any issues with the Notify feature since the new forum upgrade.
Check the Profile > Modify Profile > Notifications to see if you have set any notifications. This also shows when notifications are delivered, e.g. only once for a new post not for every new post since you last visited.