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 = geometry;

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

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