It can be easy to become overwhelmed by iOS notifications particularly if you have chatty friends or apps. In iOS 12, Apple corralled notifications by grouping them into stacks so you no longer see an endless screen of alerts. To expand a stack of notifications on either the Lock screen or in Notification Center (swipe down from the top of the screen), tap the stack. Once you’ve expanded a stack you can tap Show Less to restack it, tap the X button to remove the entire stack or tap any individual notification to open it.
By default, iOS 12 groups notifications intelligently which might entail separate stacks for different Messages conversations. If that’s still too much you can go to Settings > Notifications > App Name > Notification Grouping and tap By App to collect every notification from the app into the same stack.