As little as possible
Most horses I don't need to deworm at all. I work with the pastures and do FECs to know what horses need deworming, and then uses dewormers only to stop eggs from contaminating the pastures.
FEC in the spring, and those who need deworming gets moxidectin every 12 week (2-3 dewormings that year, depending on how long it takes for the snow to come), or ivermectin every 8 weeks, or pyrantel pamoate every 4 weeks. I rotate one year at a time. If we get a horse that shows positive for tapes, we use a dewormer containing prazikvantel one time.
All the stables I worked with (regarding parasites) have gone from massive deworming down to 10% of the horses needs regular deworming, in just 2 years. Parasitefree pastures are quite easy to accomplish if you put your mind to it. And that WITHOUT the deworming.