By default, Zakeke hides the quantity selector in the cart for customizable products . This is because you may activate pricing rules in Zakeke that are based on quantities, meaning that the correct application of the price is done by Zakeke within the customizer and if then customers change the quantity in the cart the price won't be valid anymore.
If you do not have any pricing rule in Zakeke that is based on quantity, you can still enable the quantity selector in the cart for customizable products by adding the following code in the <head> tag of your theme.liquid file. Please note that you must also disable the premium photos functionality on all your products to have the correct price be correctly calculated.
<script>
window.zakekeEnableCartQty = true;
</script>