Normally, you would just enable "Let buyer edit quantity" in the product's settings and let them enter their quantity in the cart after they add it. However, if you want your Add to Cart button to pass a predetermined quantity to the cart, you can add &quantity=2 to the button's href= URL, like so:
<a href="https://www.e-junkie.com/ecom/gb.php?c=cart&i=XXXXXX&cl=178295&ejc=2&quantity=2" target="ejejc" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this);"><img src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0" alt="Add to Cart"/></a>
If you also want to prevent the buyer from editing that quantity in the cart, you can leave "Let buyer edit quantity" in the product's settings, so whatever quantity the button adds to cart would be the only quantity they can order.
If you need to put a Quantity selection menu in your page next to the Add to Cart button, that's rather more complicated to explain and set up, so just reply to let us know if that's what you actually need.