The amount of concentrates you feed depends utterly on factors such as the breed you keep, what other feed they are getting, the stage of gestation of ewes, the type of concentrate and so on. For my primitive breeds, on hill land, a small acreage, frozen ground, with ad lib hay and licky buskets, and running with the tups, we give them about a couple of ounces per head of Tup & Lamb coarse mix per day. Because of where we are, we will continue at about that level until 6 weeks before they are due to start lambing, but doubling the amount if there is deep snow lying for more than a day. By lambing time, we will be giving them 4 ounces twice a day, maybe more depending on how they look.
For bigger non-primitive breeds, that amount will be much greater, but others can tell you how much. Also, some people will be using ewe pencils, which can be bought in two different protein values, for in-lamb and non breeding animals (I think 18% and 14% respectively). We don't use pencils just because the coarse mix looks so much more appetising

So the best thing for your particular sheep is to enquire from your breed society, or the manufacturers of the feed.
Another way of judging how much to give is empirically - in other words, you observe carefully how effective what they are getting is. If they appear to be in good condition and are not ravenously hungry, or getting thin, but eat all you put down within 5 or ten minutes, then you have got it about right. Sheep are not likely to overeat if they have all they need in front of them. If all they had was concentrates then they would probably eat too much, but as long as they have hay to 'fill their bellies' and plenty of water in addition to licky buckets and the concentrates they will be fine.