Hi,
Like that, I would recommend you to make two different products.
One product associated to a pack with a quantity of 5 and one product associated to a pack with a quantity of 6.
Using HikaShop, you can also use the plugin "group after purchase" in order to mark a customer after his bought the first time the item.
So using his new group, he won't see the product with 6 but he will see the product with 5.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.