Get number of widgets in a sidebar and add first and last widget class

functions.php

<?php
function cur_target_sidebar_add_classes_to_params($params) {

    global $my_widget_num; // Global a counter array
    $sidebar_id = $params[0]['id'];

    if ( $sidebar_id == 'sidebar' ) {

        $registered_widgets = wp_get_sidebars_widgets();
        if(!isset($registered_widgets[$sidebar_id]) || !is_array($registered_widgets[$sidebar_id])) { // Check if the current sidebar has no widgets
            return $params; // No widgets in this sidebar... bail early.
        }

        $number_of_widgets = count($registered_widgets[$sidebar_id]);
        $rounded_number_of_widgets = floor(12 / $number_of_widgets); //Rounds number of widgets down to a whole number

        if(!$my_widget_num) {// If the counter array doesn't exist, create it
            $my_widget_num = array();
        }

        if(isset($my_widget_num[$sidebar_id])) { // See if the counter array has an entry for this sidebar
            $my_widget_num[$sidebar_id] ++;
        } else { // If not, create it starting with 1
            $my_widget_num[$sidebar_id] = 1;
        }

        $classes = 'span' . $rounded_number_of_widgets;

        if($my_widget_num[$sidebar_id] == 1) { // If this is the first widget
            $classes .= ' first-widget ';
        } elseif($my_widget_num[$sidebar_id] == count($registered_widgets[$sidebar_id])) { // If this is the last widget
            $classes .= ' last-widget ';
        }

        $params[0]['before_widget'] = preg_replace('/class=\"/', 'class="' . $classes . ' ', $params[0]['before_widget'], 1);
    }

    return $params;
}
add_filter('dynamic_sidebar_params','cur_target_sidebar_add_classes_to_params');

test.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;
}
?>



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>', 
  ) );