How to limit the number of tags displayed within the Tag Cloud Widget ?

You can override the number of tags to show by inserting this filter within your child theme functions.php file.

function xt_child_widget_tag_cloud_args( $args ) {
    $args['number'] = 10;
    return $args;
}
add_filter( 'widget_tag_cloud_args', 'xt_child_widget_tag_cloud_args' , 999); 

Feel free to change the number to your needs.