Hello,
You should not use the default feature of HikaSerial but use a plugin like "serial per order".
But that plugin will send X serial per order, not per different product in the cart.
Afterwards, you can use that plugin has a base and create your own custom plugin for your specific requirement.
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.