var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(mutation) {
if (mutation.target.classList.contains('boxes')) {
var amountBoxes = mutation.target.childElementCount;
console.log(amountBoxes);
}
});
});
observer.observe($('.boxes')[0], { childList: true });
// Добавляем новый элемент
$('.boxes').append('<div>Новый элемент</div>');