If you have no other place to take sheep who need treatment, and/or if you plan on breeding your sheep, then you would find a shelter of some sort useful, yes.
The usual rule of thumb is 5 commercial-size sheep or 7 primitives per acre, but it depends on the ground. Very wet ground, very cold or exposed ground, would be less. Very lush ground in a warmer part of the country might do more.
You also need to think about where you will get their winter forage. If you need to make hay for them off this same ground, then you will need to shut part of it up for some months for the grass to grow and be cut and baled. And you'll need somewhere to store the hay, too, of course.
If you are planning to breed, then you also have to think about followers. If you choose sheep which fatten in one summer, then by the end of the summer you will have very nearly 3 times as many sheep as you started with, and then back to your starting number as you send that year's lambs away. If you choose a breed which needs two summers to grow, then in the second summer you will have nearly 3 times as many sheep as you started with at the
beginning of the summer, and all that year's lambs growing on through the summer. It adds up really quickly!