Ok, I found a way via animationComplete callback. This is my code:
JS:
jQuery(function(){
var total = jQuery('.slides_container img').length;
$("#slides").slides({
animationComplete: function(current){
jQuery(".current-slide").html(current);
}
});
jQuery('.total-slides').html(total);
});
HTML:
<span class="current-slide"></span>/<span class="total-slides"></span>