It’s not possible to redeem more than one single key at a time, and on top of that, there are limitations to how frequently you can redeem codes.
I buy assets from Humble Bundles sometimes, and they contain multiple keys.
Redeeming 28 individual keys in your system is already a nightmare, and you are making it even worse with anti spam measures…
Either do what Steam does and let me enter several codes and redeem them all at the same time, or do what Unity does and make it possible to create one single bundle key that contain several different asset store keys.