ArcGIS Pro 3.4 API Reference Guide
ArcGIS.Desktop.Mapping Namespace / BasicFeatureLayer Class / SelectionCount Property
Example Version

SelectionCount Property (BasicFeatureLayer)
Gets the number of features selected in the layer.
Syntax
public int SelectionCount {get;}
Example
Count the features selected on a layer
var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
var noFeaturesSelected = lyr.SelectionCount;
Requirements

Target Platforms: Windows 11, Windows 10

ArcGIS Pro version: 3 or higher.
See Also