SelectionCount Property (BasicFeatureLayer)
Count the features selected on a layer
var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
var noFeaturesSelected = lyr.SelectionCount;
Target Platforms: Windows 11, Windows 10, Windows 8.1
ArcGIS Pro version: 2.0 or higher.