When we typically get notified in the form of spam or self-explanatory repetitive messages, we may forgo deleting or tapping the message at all. These begin accumulating and suddenly there's thousands of unseen messages drowning the important ones. Instead of tapping messages one-by-one, there's a way to mark all of your messages as read, along with selecting specific messages.
How to mark all messages as read
Go to the Messages app
Tap 'Edit' in the top left corner
Tap 'Select Messages'
To mark all messages, tap 'Read All' on the bottom left
To mark specific messages as read collectively, simply tap them, when you're done, tap 'Read' on the bottom left.