Unfortunately, I do not have enough reputation to comment on stackoverflow, so this is coming as a 'answer'.
I'm not sure of any way to do this with Weebly. The carts count/content seams to load last, so any JQuery ect.. wouldn't be able to count the contents. Even using something like var str = $( "span#wsite-nav-cart-num" ).text();
returns a dash - and not the actual number that appears in the CART (1).
I'm not sure why you would want this anyway, but that's up to you. I would suggest submitting a features request to Weebly.
P.S.. Weebly has community.weebly.com where your Weebly questions might get some better results,.. that is, unless someone directed you here.
EDIT:
Actually,.. After seeing your comment, it popped into my head, you can do a 'touchstart mouseover' event. Try this, in your Settings > SEO > Footer Code.
<script>
jQuery(function() {
var $ = jQuery;
$(document).on('touchstart mouseover', 'a#wsite-com-minicart-checkout-button', function(e) {
var totalCount = $( "span#wsite-nav-cart-num" ).text();
var totalNeeded = 12;
if (totalCount < totalNeeded) {
var totalItems = totalNeeded - totalCount;
$("a#wsite-com-minicart-checkout-button").click(function(event){
event.stopImmediatePropagation();
alert('We require ' + totalNeeded + ' items in order to checkout. You currently have ' + totalCount + '. Please add ' + totalItems + ' more items to the cart.');
return false;
});
}
});
});
</script>
The downside, something with the cart causes the alert to appear twice. And while it should work with a Responsive Theme(your current Theme), this wont work with Weebly's classic mobile Themes.
In the end, it will still be better if Weebly builds this into their system. Hope that helps!