It sounds like the rooster is causing the damage when he mates with them. I don't consider that an aggressive rooster. There could be many different things causing it, but it sounds like your rooster's behavior is a huge contributing factor. If it were just one or two hens affected, the hen's behavior could be the problems more than the rooster's, but with several, the suspicion goes toward the rooster.
There is no magic ratio of hens to rooster that causes this or prevents this. More hens per rooster might help, but some people have three roosters with two hens and never have this type of problem. Others have 18 to 20 hens with one rooster and have this problem with some hens.
If they are young, they may grow out of it. A young rooster often has really over-active hormones and has not yet reached the maturity where the hens respect him enough to willingly squat for him. He winds up forcing them. Compounding the problem with young chickens, often young pullets are not ready for mating so they resist. The rooster's hormones cause him to force himself on them. As I said, they might grow out of it. But they might not. Depends on their personalities. Often the rooster's technique improves with age too.
Some things that could help. You can trim the rooster's spurs and claws. There are different methods. I prefer the dremel tool. Just wrap him in a towel and cut. As long as you don't go too deep and hit the quick, it is just like trimming your fingernails. He does not even notice. You are not trying to remove the whole spur or claws. Just try to blunt them so they are not sharp. I think the claws do the most damage of this type, but the spurs can cause some damage too. The danger is that the rooster will cut the hen and cause an open wound.
You can also make chicken saddles for the hens. It won't help the back of their heads, but it will protect their backs from those cuts and scratches. I'll give you a link to a pattern. It's really not that hard to make them. I find some of the time the snaps (I use the hammer on kind) don't hold real well. I've had better luck with the buckle type, then put in a couple of stitches after they are on to keep the buckles from slipping.
Chicken Saddle Patterns
https://www.backyardchickens.com/forum/viewtopic.php?id=407575
Good luck. What you are seeing is not unusual and is not necessarily real serious, but it can become serious. It needs addressing.