Even though a shetland can survive on less grazing/less quality, doesnt mean that they will eat less

they will just eat all of it and become fat, then when the grass has run out they will be hungry!
So if it is 4-5 sheep to the acre, and all together your land is half an acre, I wouldnt have more than two sheep on it!, specially in the winter. If your grass is very good (again weather/time of year permitting) I would say mayyybe 3 sheep with hay in the winter. Some shetlands will refuse to eat hay and prefer to dig through the snow to eat frozen grass instead!
Shetland sheep do make lovely lovely pets and mine are very tame, they will however consume all of your trees/hedges/plants aswel as the grass, I wouldnt have lambs on this small amount of grass though, as then the ewes will need more grass and the lambs eat it aswel!
If they are just pets then two shetland wethers would be ideal!
hope this helps