Depends on your bank balance! They are like gold dust at the moment, apparently they were being snapped up by the Chinese during the Olympics as they eat them!
We have not been able to get our usual supplies, and they have gone up by about �6 for 50, thats if you can find any. We now sell a net of 50 for �20, whereas they were only �15 a couple of months ago.
As to feeding them, you could give one a day, depending on the size of your dog, although some dogs make them last longer. They will do no harm at all, and they help them to get rid of their baby teeth and push the adult ones through nice and strong. By far our biggest seller.