Where to add the JQuery code

This is wonderful thanks, I have another question if you could please help me, In this same code can the limit # be variable. I mean can I create a drop down field with numbers and the number on that field is the limit #. How would that work?

I was thinking something like this:

<script type="text/javascript">
jQuery.noConflict();
jQuery(document).ready(function($) {
$.fn.limit = function(n) {
var self = this;
this.click(function(){ return (self.filter(":checked").length<=n); });
}
$("ul#input_1_15 li input:checkbox").limit("ul#input_1_15 li input:select");
});
</script>

Thanks Rob, I saw the working example but if I change the number to 4 I am still limited only to check 3 checkboxes. I was thinking on adding a dropdown with the number. I'd like to explain the reason why I am trying to do this, I am building an "order Pizza" the problem is that there are so many variations. Like first number of pizzas to order, second each pizza can be different then each one has a different price after that you need to select the number of ingredients (each extra ingredient also costs extra but it is a different price depending on the size of pizza) Now here where I ma using the limit checkboxes I am adding a dropdown asking for the number of ingredients and once they select the number then they will only be able to check those boxes. 4 ingredients will only allow them to select 4 checkboxes. Which takes me to the next problem every ingredient can be on the right half, the left half or the whole pizza. If one is selected as half then they have the right to add another half (it has been a nightmare for me honestly)