How many times is it appropriate to receive the hair loss treatment once a week?

I went to the hair salon and froze on the spot when I was told I had developed a circular patch of hair loss the size of a 500-won coin at the bottom of the back of my head. When I fumbled around with my hands, I felt nothing but smooth skin with not a single strand of hair, and it gave me goosebumps. It would be a relief if it just stopped at one spot, but I read that it spreads and spawns, so I can't concentrate on my daily life because I have to check the back of my head with two mirrors every day. I was told that it is due to an immune response and that I need to go to the hospital and get a steroid injection directly into my scalp to stop it from spreading. However, I'm scared because I saw reviews online saying that the injection site can cause a side effect where the scalp sinks in. I need tips from those with experience on how many days it usually takes for fine hairs to start growing back after the injection, and how many times a week is appropriate to get the injections safely without any sinking.