Genesis: Add classes to footer widget areas container, depending on the configured number of footer widget areas.

functions.php

<?php

add_filter( 'genesis_footer_widget_areas', 'gmj_add_footer_widgets_classes', 10, 2 );
/**
 * Add classes to footer widget areas container, depending on the configured number of footer widget areas.
 *
 * @param string $output         Existing footer widget areas container markup.
 * @param int    $footer_widgets Number of configured footer widget areas.
 * @return string Possibly amended markup for footer widget areas container.
 */
function gmj_add_footer_widgets_classes( string $output, int $footer_widgets ): string {
	$classes = [
		'',
		'col col-xs-12 center-xs',
		'col col-xs-12 col-sm-6',
		'col col-xs-12 col-sm-6 col-md-4',
		'col col-xs-12 col-sm-6 col-md-3',
		'col col-xs-6 col-sm-4 col-md-2',
	];

	array_walk( $classes, function( $class, $key ) use ( $output, $footer_widgets ) {
		if ( $key === $footer_widgets ) {
			return str_replace( 'widget-area"', $class . ' widget-area"', $output );
		}
	});

	return $output;
}