Jquery Disabling individual buttons within search results that all have the same classes applied
$("button.addtocart").click(function(e) {
    // fetch <input name='stock_amount'> within the clicked buttons form element
    var stock_amount_input = $("input[name='stock_amount']", $(this).parent());
    // fetch <input name='_qty'> within the clicked buttons form element
    var qty_input = $("input[name='_qty']", $(this).parent());
    // maybe you want to do some check here
    var new_amount = stock_amount_input.val() - qty_input.val();
    // set the new calculated value in the stock_amount input
    stock_amount_input.val(new_amount);

    // if the new_amount is less than 1 we dissable the clicked button
    if(new_amount < 1) {
        $(this).attr("disabled", "disabled");
    }
});

var nodes = $("form");
for(var i=0; i<nodes.length; i++) {
    var node = nodes[i];
    // do something
}