Oh, now I see. It appears you're using customized button code to let buyers enter a quantity on your page before they add an item to their cart. Normally, buyers would edit quantity in the cart itself after they add an item, so that's where I got confused. Adding a value="" attribute to your quantity <input> tag would prefill that field with whatever value you specify, e.g.:
<input type="text" name="quantity" size="2" value="2">
