How do I track and limit a User to choosing any 6 docs from a container?

I have an issue that I’m trying to solve:

A user buys a limited access package that allows them to choose any 6 out of 52 possible products.

Problem: How do actually limit them to any 6? Do I need a new snippet that tracks which 6 they’ve chosen and adds those choices to their User account?

Those 6 are not predefined. They can be any 6 of 52.

Keep a count in the SESSION. Add an increment to a SESSION variable to the add-to-cart snippet.

That works at the beginning of the transaction process.

Will it work after the transaction when they login and start selecting the videos from a list?

Thank you.