I have always gone by the rule of 2 acres per horse for grazing. It will quickly become overgrazed if not managed properly. Can you split it so that they are on half for 30 days then on to the next section for 30 days?
I keep 4 horses on a 2 acre dry lot during the winter. I turn them out for a few hours every day all year on a 7 acre pasture. I plan to split the pasture into 4 equal paddocks and do a 30 day rotation during the grass growing season. Just waiting to win the lottery so I can afford to add the extra fence
Until then I only turn them out for 3-4 hours a day then back to the dry lot.