Talk to the owner. If they are ok they are ok. X sheep to the acre, x grass length, it is all meaningless if the sheep are suffering - and equally if they are not. They are just guesses, guides, indicators.
Mine have nothing but moss, reeds and the wind atm, some are ewe lambs, some are 8 yo brokers. A roadside look or internet opinion would say they do not have enough. They are all happy as you like and if anything over fat as they are not due until April.