We keep 2 large horses on 1 acre so I'm sure you'll be fine. I have to say we do make sure we poo-pick every day and the horses have haylage nearly all year to supplement their grazing.
We have two horses (17.2 and 16hh) and 2 mini shets on 5 acres and we only just have enough. Fields are weed killed, rolled, harrowed, rested and strip grazed. We are usually grassless after late Jan
I have 2 biggies 16'2 + and a pony on 3 1/2 acres and they have more grass than they can eat and they are out 24/7 through the summer and in by night in the winter.
We have 3 on 6 acres (1 x 15.1 and 1 15hh yearling plus 1 Section A) of poorish grass, very heath type grazing. It is enough but wouldn't really want more mouths on it. They are pretty much out 24/7 and supplemented with hay winter through late spring.