Hi,
There is no central option. However, there are two ways you can do that easily for all the products:
1. You can create a massaction to update the product_max_per_order field of the products all at once:
www.hikashop.com/support/documentation/167-massaction-form.html
You can even configure the mass action with a trigger on the product creation so that you won't have to set it for new products.
2. You can edit the file "quantity.php" of the view "layouts" via the menu Display>Views and change the line:
$max_quantity = (int)$min_quantity * 15;
However, this is only for the product listing and product page. For the checkout, cart, and legacy views, you would have to change that same line in other view files:
checkout/show_block_cart.php
checkout/cart.php
product/show_block_characteristic.php
product/show_quantity.php
product/add_to_cart_listing.php
You might not need to change all of them. It depends on how your shop is configured. At least show_block_cart.php and quantity.php will be necessary.