Do you mean how many to keep all the grass eaten? 2-4 won't eat down 7 acres and you would be better to keep that number in a smaller area of your land, partitioned off, so they graze what they have well down, before you move them onto the next bit.
From the point of view of whether the sheep will be content, anything above one will be fine, so they can have a mini flock. Practically, it's better to have at least 3, just in case one dies.