In cats, when scratching becomes noticeable, usually a skin parasite or allergy is involved.
You need to watch just how much scratching you are really seeing and when you have a pretty clear idea, there are two things you can do. You can take kitty to the vet for a skin scraping, which would reveal the presence of skin parasites you can't see with the naked eye, or you can change the diet to one that concentrates on avoiding all the known serious allergens and see if that makes a difference.
While you are watching scratching, also watch the amount of licking your kitty is doing. Like scratching, licking is also a clue that tells us something is going on and they don't feel comfortable.
Indoor cats can be allergic to any food ingredient, pollens, and materials such as wool for instance, in the home. Many things.
If a lot of scratching is going on, as a safety precaution it's good to have a skin scraping done first to rule out skin parasites and then one can start working on the diet in earnest.
Also, from looking at the skin the vet can see something our untrained eyes overlook.
Basically, you shouldn't see so much scratching that you start paying attention to it. When there is that much of it, there is a problem, and you need to start looking for the cause.