itch.io does not offer the option to limit the number of times a file can be downloaded after a user purchases a product. This is intentional, as such DRM measures often harm end-users and are ineffective at preventing piracy.
If you, as a publisher, go out of your way to add restrictions about how users can access purchased goods, itch.io may refund transactions and limit your ability to receive payouts. As a publisher we expect you to ensure that your customers are able to access the products that they have paid for.