When creating a new Promotion, there are a number of limits that can be set.
How many set of X can be redeemed?
This is basically, is there a limit on the total number what ever you're giving away. It could be;
a) You're offering unlimited $10 vouchers (but 1 per customer);
b) 500 Movie tickets
Do you want to limit customer participation?
This related to how many times a unique customer to redeem this promotion. For all promotions that are gifting something or are a new-memebr incentive promotion this should be set to Yes and then One.
If however, a unique customer can redeem this particular promotion more than once, you can then set a limit on how often they can actually redeem. Often, client will use this to say the customer can Facebook Check-in an unlimited number of times, but can they can only actually access/use the promotion one every 7-days.