This should give you an XML object with all of the metadata being presented in the metadata panel of Premiere.
Inside there is a child "<premierePrivateProjectMetaData:Column.Intrinsic.VideoUsage>".
It seems it is a string though, but this you can split as well by above string to give you the number.
You can also wrap it into an XML object
var theXML = new XML(app.project.rootItem.children.getProjectMetadata());
and eventually you will find the correct child.
var usageCount = theXML.child(0).child(0).child('premierePrivateProjectMetaData:Column.Intrinsic.VideoUsage');
Depends on what you're trying to achieve.