This example handles the CustomDrawNodeIndicator event to display numbers within node indicator cells:
private void treeList1_CustomDrawNodeIndicator(object sender, DevExpress.XtraTreeList.CustomDrawNodeIndicatorEventArgs e) {
TreeList tree = sender as DevExpress.XtraTreeList.TreeList;
IndicatorObjectInfoArgs args = e.ObjectArgs as IndicatorObjectInfoArgs;
args.DisplayText = (tree.GetVisibleIndexByNode(e.Node) + 1).ToString();
e.ImageIndex = -1;
}