I think that would be a hard one to answer since "possessed" would be a moving target that changed at any time during the war. The number was affected by lost cannon, captuered cannon, exploded cannon, new production, and old worn out ones being sent back to be melted to make new ones. I just got home from work and am too tired to look at the moment but think there might be some sort of generalization in one of the books on artillery such as "Artillery and Ammunition of the Civil War" by Warren Ripley. I'm sure the snapshot in time would vary greatly as the war progressed with 6-lbers falling by the wayside and rifled cannon coming into more widespread use.