Actually it is possible to do without losing your changes but it’s complicated. What you’d need to do is follow the instructions here to unregister the default widget and then register your own: https://theeventscalendar.com/how-to-completely-customize-widgets/
In your own copy of the widget you’ll want to point it to a different admin-view file which controls how many numbers are in the drop down for the widget.
The only problem with this approach is that if there are significant updates to the core code, you may have to reverse engineer your changes you made to match up with the new core code…
Good luck!