Edit in JSFiddle

$('.count').each(function () {
    $(this).prop('c',0).animate({
        ｃ: $(this).text()
    }, {
        duration: 4000,
        easing: 'swing',
        step: function (now) {
            $(this).text(Math.ceil(now));
        }
    });
});

span{color:red;font-weight:bold;}

あなたは<span class="count">20000</span>人目のお客様です！