I don't quite understand what you're trying to do. I mean, I understand what you're suggesting, but I don't understand how you want to integrate it into Itch.
In Itch, a purchase gives that buyer access to a single key. If you use the external keys dashboard, it's assumed that a buyer can only claim one key from the stock.
I'm not an expert on keys, but what you're suggesting doesn't sound like it could be implemented with Itch's existing tools, as they aren't designed to work that way.
EDIT: A clarification: Creating keys per bundle generates a group of keys that you can manually distribute on another platform, for example, to integrate your Itch sales into a bundle outside of Itch.
This option does not work for selling groups of keys when someone buy your project.
EDIT2:
I reread and think you're referring to the "Unlock price" option.
What that field does is indicate which Tier will have that group of keys, but that only works if your project uses per-file pricing.
Example:
Your project costs $5 and has three files.
The main file does NOT have a per-file price. So anyone who purchases your project will have access to the main file.
The second file has a per-file price of $8.
It's not enough to purchase your project; to download this file, people must have paid $8 or more at the time of purchase.
The third file has a per-file price of $12.
You generate a group of keys and set the value of 10 to "unlock price."
Anyone who claims your project using one of those keys will be equivalent to purchasing your project for $10. That is, they will be able to access the second file (tier $8), but not the third (tier $12), because the value of the key is equivalent to $10.