They don't need scratch. There are a wide variety of feeds sold as scratch. Some are fairly nutritious and make a good treat, and some are all or mostly corn, which they love but which doesn't provide a lot of nutrition by itself, kind of like chicken ice cream. All they need is their pellets or crumble -- grower, flock raiser or layer -- and fresh water. You can give them vegetable scraps and many leftovers as treats if you wish. I buy layer pellets and BOSS, or black oil sunflower seed. The love BOSS and it's a lot more nutritious than corn, but then outr scratch is mostly cracked corn. I still don't give a lot of BOSS, a handful or two in a day at most for 9 hens.
I have no idea how much to feed, sorry, because I feed free choice.