It sounds like a wonderful plan. Many of us can only rear pigs outdoors in summer as it’s too wet and boggy in winter.
Although it sounds like a lot of land, as I’ve very little experience of pigs in winter other than in fields (and here on mud all winter), I might suggest try it with 10 or 12 the first year, just to see. If the acreage is a little tight, they could do a great deal of damage to the tree roots. If you’re planning on feeding well twice a day, this would be less of an issue, but if you’re expecting them to acquire some of their ration from the forest floor, they will rootle with serious intent. (They’ll rootle, eat roots and damage trees anyway, just a lot more so if they’re not getting well fed twice a day.). So if it were me, I’d build up to 20 over a few years, rather than go the whole hog (sorry, couldn’t resist) from day 1.
Whatever you do do, good luck - and please tell us how it goes!