I used this:

function wpb_countx() {
    wp_enqueue_script(\'jquery\');
    ?>
        <script>
   jQuery(function($) {
    // configure
    var comment_input = $( \'#commentform textarea\' );
    var submit_button = $( \'#commentform .form-submit\' );
    var comment_limit_chars = 1400;
    // stop editing here

    // display how many characters are left
    $( \'<div class="comment_limit_info"><span>\' + comment_limit_chars + \'</span> zbývá znaků</div>\' ).insertAfter( comment_input );

    comment_input.bind( \'keyup\', function() {
        // calculate characters left
        var comment_length = $(this).val().length;
        var chars_left = comment_limit_chars - comment_length;

        // display characters left
        $( \'.comment_limit_info span\' ).html( chars_left );

        // hide submit button if too many chars were used
        if (submit_button)
            ( chars_left < 0 ) ? submit_button.hide() : submit_button.show();
    });
}); 
        </script>
    <?php
}
add_action(\'wp_footer\', \'wpb_countx\');