Depends on breed, condition, grass, number of lambs, etc, etc.
A Mule or Texel rearing twins on not great grass will be needing 2lb per day.
A primitive rearing one lamb on the same ground would not need anything.
Whatever you are feeding, keep a check on the ewes' and lambs' condition, and if they're fat, reduce the cake; if they're thin, then up the cake. (Assuming they don't need worming or flukeing.)