Some time ago i searched for a way to limit elements on a sortable box (i used jQuery Interface). For example, i had two boxes and one of them needs no more than three items. After i searched and switch net upside down, i ask directly to the creator or Interface and he gave me a simple (but still efficient) solution:

$('.sort').Sortable({
accept : 'sortItem',
activeclass : 'sortableactive',
	hoverclass : 'sortablehover',
	helperclass : 'sorthelper',
	opacity: 	0.5,
	fit :	false,
	floats:	true,
	onStart: function(){
	console.log($('#big .sortItem').size())
	if ($('#big .sortItem').size() == 3) { //max of items
		console.log($('#big').get(0).dropCfg.a)
		$('#big').get(0).dropCfg.a = 'dummyRandom';
	} else {
		$('#big').get(0).dropCfg.a = 'sortItem';
	}
}
});

Demo here (click!).