If you want them at home for lambing, then think about how many ewes with twins you can comfortably manage in your 4.5 acres. Depending on the ground, 20 might be pretty much max, I would think. I think I'd probably start with 10 - after one lambing, even if you only like half the girls, you'd be up to 15 keepers already.
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On choice of breed, if you think there's any chance you or your missus might decide to spin
or make other use of their fleece, don't choose Easy Care!