Как вариант...
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$(function(){
$('.add_new_ing').click(function(){
var n=($(this).data('operation')=='minus')? -1: 1;
var o=$(this.parentNode);
var ocnt=o.find('.count_ing');
var val=+ocnt.text();
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};
ocnt.text(val);
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<div class="pizza_add_ing">
<ul class="add_ing_list">
<li data-ord="10" data-id="1">
<span class="add_new_ing add_ing_minus" data-operation="minus">-</span>
<span class="count_ing"></span>Сыр
<span class="add_new_ing add_ing_plus"data-operation="plus">+</span>
</li>
<li data-ord="15" data-id="2">
<span class="add_new_ing add_ing_minus" data-operation="minus">-</span>
<span class="count_ing"></span>Томаты
<span class="add_new_ing add_ing_plus"data-operation="plus">+</span>
</li>
<li data-ord="20" data-id="3">
<span class="add_new_ing add_ing_minus" data-operation="minus">-</span>
<span class="count_ing"></span>Курица
<span class="add_new_ing add_ing_plus"data-operation="plus">+</span>
</li>
</ul>
</div>
</body>
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