https://www.cnet.com/news/group-notifications-help-you-streamline-your-attention-on-ios-12-at-wwdc/

Your notifications are about to get simpler. At WWDC, Apple showed off group notifications. 

If multiple people send you an iMessage, instead of all of those messages clouding your notifications tray, they'll be pulled together into a single notification letting you know you have multiple messages.