Depending on where you live and your weather will depend on what you need to use. I only use 2 a year. We live in a vary dry climate and we don't carry that amount of bad things that other places have. You also need to look at your horses age and condition. Some horses that are under weight need to be watched with some brands.
Like freemare said, it depends on your climate and the time of year. It also depends on the environment. Tape worms, for example, require a 2nd "host" that is found in grass...so there is a larger threat in lush grasses. However, this doesn't mean you shouldn't ever deworm for them in an arid environment. Some parasites do not thrive well after they are pooped out in heat, some in cold. These are a few of the reasons why a schedule needs to be tailored to the specific horse(s) to be effective.