public int MaximumNumberOfPoints {get; set;}

{
  // must be on MCT 

  var clusterFilter = new LasPointClusterSelectionFilter();
  clusterFilter.VisiblePoints = true;
  clusterFilter.ClassCodes = new List<int>();  // empty list means all classification codes
  clusterFilter.FilterGeometry = polygon;

  clusterFilter.SearchRadius = 0.5; // meters
  clusterFilter.MaximumNumberOfPoints = 500;

  var clusterSelCount = await lasDatasetLayer.SelectAsync(clusterFilter, SelectionCombinationMethod.New);
}