It depends on what type of softener you have. Some are totally full but most are 1/2 full.
1/2 means the resin bed is about 1/2 the height of the softener tank. We may be able to help if you can tell us the name of the softener manufacturer.
You can buy resin for most any Water Conditioning Dealer. You will have to know what type of resin is in the unit. There are several varieties. The Dealer may know which by looking at the unit.
I may be able to tell you which if you can tell me the manufacturer and the Model Nr.