Coupons must be associated with coupon code promotions rules so that a promotion is triggered when customers enter a specific coupon code at checkout. The coupon can be used multiple times and it is considered valid until its usage count exceeds the threshold set by its usage limit. About that:
Set the usage_limit attribute to 1 if you want the code to be used just once.
Set the customer_single_use attribute to true if you want it to be used just once per customer.
Coupon codes minimum length is 8 characters, maximum length is 40 characters.