The one-horse-per-two-acre-rule.
Yes. Except that the gentleman doesn't actually have two acres of pasture available for each horse. Barn, house, garden, driveway, parking, equipment shed, well, manure pile, chicken coop and/or goat-shed, and so forth have a surprisingly large footprint and take up a lot of that ten acres. You end up having only five or so acres left. It's very easy for five or six horses to turn five or six acres into an over-grazed dustbowl in summer and a mudpit in winter. Oh, and he's planning to spread manure on his fields, which means that they're not going to be continuously available, no matter what the worming schedule is. That's not the best pasture management.
Just a suggestion, it's not really any of my business. I concede that he knows his own property and his part of the US as I do not, and my knowledge of pasture maintenance is limited to the four areas of the US where I've owned property.