So is it 6 horses on 20 acres 24/7 or is that 7 part of the total, and 6 horses on 13 24/7? In any case, my inclination would be to create two dry lots, ideally three. But two could work. Those would be small. I have a dry lot of a quarter acre that works for 2 drafts and a pony. They can stay in it for days if required, not happily, but they can. I then have a 1 acre field (flat, dry, mostly saved for winter or rainy weather) and two 3 acre fields. In general they only get 12 hours a day on those. They spend 8 to 14 hours in the dry lot. And I rotate according to the grass and the weather. One field is dry, the other is swampy, I use that to my advantage. In wet weather I try to put them on the dry field. In dry weather on the wet field.
You will be feeding hay. I would recommend three small dry lots, for ease of catching horses, winter weather issues, and letting fields rest as needed. By having well fenced dry lots that they spend much of their time in, you can actually reduce the pressure on the other fences. (in my experience)
When I have a solid cold winter, I can and do let them out on the bigger fields. But if I have to (I am in CT) I can use the small, flat acre paddock to let them stretch their legs during the day, and keep them in the dry lot at night. One warm muddy winter that was a stretch from January to April. Then I rested that acre and reseeded it. But I did not beat the other six up!