Get number of active WordPress widgets in a widget area

functions.php

<?php

/**
 * Count number of widgets in a sidebar
 * Used to add classes to widget areas so widgets can be displayed one, two, three or four per row
 */
function slbd_count_widgets( $sidebar_id ) {
	// If loading from front page, consult $_wp_sidebars_widgets rather than options
	// to see if wp_convert_widget_settings() has made manipulations in memory.
	global $_wp_sidebars_widgets;
	if ( empty( $_wp_sidebars_widgets ) ) :
		$_wp_sidebars_widgets = get_option( 'sidebars_widgets', array() );
	endif;
	
	$sidebars_widgets_count = $_wp_sidebars_widgets;
	
	if ( isset( $sidebars_widgets_count[ $sidebar_id ] ) ) :
		$widget_count = count( $sidebars_widgets_count[ $sidebar_id ] );
		$widget_classes = 'widget-count-' . count( $sidebars_widgets_count[ $sidebar_id ] );
		if ( $widget_count % 4 == 0 || $widget_count > 6 ) :
			// Four widgets er row if there are exactly four or more than six
			$widget_classes .= ' per-row-4';
		elseif ( $widget_count >= 3 ) :
			// Three widgets per row if there's three or more widgets 
			$widget_classes .= ' per-row-3';
		elseif ( 2 == $widget_count ) :
			// Otherwise show two widgets per row
			$widget_classes .= ' per-row-2';
		endif; 

		return $widget_classes;
	endif;
}

?>