Every notification names the account it is for. Opening it switches to that account first, and the icon’s count adds up every account signed in on the device. Signing one account out stops its notifications there and leaves the others’; a notification for an account no longer signed in on the device is shown without its words.
Notifications for each account on a shared device
A phone or browser signed in to several Kemble accounts gets each one’s notifications, and tapping one opens the account it is for.