As a rough guide, I reckon on 16 medium ewes eating a 25kg (or thereabouts) bale of hay/day.
However, I agree with Horlicks - give them half a bale to start with and let them finish it before giving them more. If you give them more than they need they will pick out all the choicest bits and leave the rest to be wasted; as once they've decided to reject it, they will have to be near starving before they will agree to finish it off.
By the way, hanging hayracks will result in a lot of waste also, as once it has fallen on the ground the hay will be trampled on and again they will assure you that they cannot possibly eat it.
The best thing is to have a system where they have to put their heads through to eat the hay. You can acheive this by putting a gate or hurdle upside down, with the hay behind, so they can get their heads through the bottom rail to eat the hay. What they drop then remains behind the barrier instead of falling at their feet to be trodden on.
To see what I mean, google "sheep hay feeder" and press
images and you should see pics of the sort of thing I'm on about.