Most Farmskins promo codes are eligible for one-time use only, meaning a user can use a promo code only a single time. This limitation ensures that each user gets a fair opportunity from the promotional offer without getting to abuse the system.
However, that being said, it can be possible that some very exclusive or very rare Farmskins promo codes might be allowed for some players to use multiple times. It has never been done before but if it does get done in the future, it would surely be historical.