Each Rewards campaign will have a finite number of users, depending on the number of claims you have configured, or the number of coupon codes you upload.

If you upload 100 unique coupon codes, then a maximum of 100 users can each claim a unique code.

If you configure for a single code to be claimed 100 times, then a maximum of 100 users can access that same code.

By default, number of claims are restricted to one claim per unique IP address. You can change this according to your own preferences through the Reward tab on campaign setup.