Validating Checkboxes By Value

I found some simple code to get a better understanding of validating checkboxes. It validates the checkboxs by their name. Is it possible to validate the checkboxes with the same name but different values? The name of the checkboxes will be an array containing the values of the checkboxes selected.

I have my html like so:
<input type="checkbox" name="stuff[]" value="1" onClick="countChoices(this)">
<input type="checkbox" name="stuff[]" value="2" onClick="countChoices(this)">
<input type="checkbox" name="stuff[]" value="3" onClick="countChoices(this)">

And the javascript where I edited "box1=", "box2=", "box3=" is like so:
function countChoices(obj) {
max = 2;

box1 = obj.form.1.checked;
box2 = obj.form.2.checked;
box3 = obj.form.3.checked;

count = (box1 ? 1 : 0) + (box2 ? 1 : 0) + (box3 ? 1 : 0);

if (count > max) {
alert("You can only choose up to " + max + " choices!
Uncheck an option if you want to pick another.");
obj.checked = false;
}}

How do I correctly create box1, box2, box3, in this case?