Normal shower water pressure is between 45 PSI to 55 PSI.
The same goes for water pressure throughout your house.
A good water pressure reading for a house plumbing system throughout your house and outside is 45 to 55 PSI.
If your water pressure is lower than 45 PSI then you may have a plumbing issue such as clogged water pipes in the street, in your yard etc.
If you have low water pressure just in your shower then there could be a restricting valve on your shower water line or your shower head could also be clogged.
If your shower head is clogged and not allowing enough water pressure through your shower then you can soak the shower head in a bag of CLR or Vinegar to clean the Calcium deposits in the shower head.
You can either remove the shower head to clean it or just tie a Ziploc bag of vinegar or CLR onto the shower head and allow the shower head to soak in the bag for a few hours to overnight.
Then after the shower head is done soaking you can run some hot water through the shower head for a few minutes to clean the shower head on out.