The default version number of the app is 1.0.0.0.

Note:
  Microsoft Store apps are not allowed to have a
  Version with a revision number other than zero.
  Therefore, the last number of the version must
  remain zero for all releases.
  Ensure that you follow Microsoft’s
  versioning guidelines.

For apps not published to the Microsoft Store, you
can set the app’s executable’s file and product versions.
The executable’s default file version is 1.0.0.1,
and its default product version is 1.0.0+1. To update these,
navigate to the pubspec.yaml file and update the
following line:

version

1.0.0+1

The build name is three numbers separated by dots,
followed by an optional build number that is separated
by a +. In the example above, the build name is 1.0.0
and the build number is 1.