strongly agree that letting to individuals will be a nightmare that yo probably don't want. Possibly let to one person and if you allow them to sublet, have a very strict contract on how many, who is responsible for maintenance of the property, removal of muckheap etc etc. 12 acres is not enough for 10 horses unless its particularly well draining and very well managed. I would check that people that come in know how to maintain grazing, too many do not-its expensive and time consuming.
I rented yards for a number of years, some I shared 50:50, some I sublet and had my own liveries. I have just bought my own place and even if it was big enough, I wouldn't entertain the thought of liveries. I was in East Lothian which isn't the cheapest-we had a beautiful brick built yard with 5 stables, feed room and tack room (no hay storage or lorry/trailer parking) which was a pain, 2 loose boxes further away, 4 post and railed paddocks with very good field shelters, a sand and rubber floodlit arena, muck heap removal and access to very good off road hacking for £640pcm. we were not responsible for repairs/maintenance (unless it was one of our horses that did it) but were responsible for paddock maintenance. Although we had 11-12 acres, it couldn't support 6 horses out at grass 24/7 all year round because of the layout of the paddocks, even though the ground was mostly well draining and not clay. We had 4 horses and two ponies on it.