Basically, there is only 1 dominant for every key, 5-1. Anything else is a secondary dominant: 7-3, 3-6, 6-2, 2-5, 1-4. You can even have secondary dominants going to secondary dominants, hence the 7-3-6-2-5-1 progression, ending with the true 5-1 dominant.
Cool stuff man, keep the knowledge flowing.