Hi,
It could be Outlook, or a mail/messaging app that you might have installed that is showing you notifications on your lock screen. The numbers will depend on the amount of stuff such as mail or chat messages that you haven't seen yet.
If you want to have it removed, just simply modify your lock screen settings by following these steps:
- Go to Start, then select Settings.
- Go to Personalization, and then click on Lock screen on the left pane.
- Under Choose an app to show detailed status, select only what you need/want. Default is the Calendar.
- Under Choose apps to show quick status, select only what you need/want.
You may also refer to this article: Personalize your lock screen, for more information.
Hope this helps. If you have any other concerns, don't hesitate to come back to us.