next_widget_id_number( string $id_base )

function next_widget_id_number( $id_base ) {
	global $wp_registered_widgets;
	$number = 1;

	foreach ( $wp_registered_widgets as $widget_id => $widget ) {
		if ( preg_match( '/' . $id_base . '-([0-9]+)$/', $widget_id, $matches ) )
			$number = max($number, $matches[1]);
	}
	$number++;

	return $number;
}