Here you go.
// https://resource.dopus.com/t/rename-script-inquiry-rename-folder-to-have-number-of-files-within-it-added-to-the-folder-name/45103
var fso = new ActiveXObject('Scripting.FileSystemObject');
function OnGetNewName(getNewNameData) {
if (!getNewNameData.item.is_dir) return;
return getNewNameData.newname + ' (' + fso.GetFolder(getNewNameData.item).Files.count + ')';
}
Save 45103.orp to
%appdata%\GPSoftware\Directory Opus\Rename Presets