Thank you for your question, it is a little tricky but....1st you do need to run two 10 inch Chloramine Blasters in a row, for system up to 300 GPD to work. It is a reaction that takes place on the carbon, and it requires contact time with the material.
I am assuming when you ask me how many gallons is one 10-inch capable of treating you are referring to Chloramines.
So, the answer is; one 10 inch Chloramine Blaster is not capable of treating chloramines. Although you can remove chlorine.