Add a class to reflect the number of widgets in a given sidebar

functions.hpp

<?php
/**
 * Get number of active WordPress widgets in a widget area
 *
 * Snippet by GenerateWP.com
 * Generated on November 26, 2018 15:47:27
 * @link https://generatewp.com/snippet/2V0V0gy/
 */

/**
 * 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;
}


// Apply the function to `register_sidebar`

register_sidebar( array(
    'name'           => 'Footer',
    'id'             => 'footer',
    'description'    => 'Footer Items',
    'before_widget'  => '<div id="%1$s" class="footer-widget %2$s '. slbd_count_widgets( 'footer' ) .'">', 
    'after_widget'   => '</div><!-- .footer-widget -->', 
    'before_title'   => '<h3>', 
    'after_title'    => '</h3>', 
  ) );

Taken from: https://generatewp.com/snippet/2V0V0gy/