Mahlon, on the more serious side, check this site: http://www.civilwarhome.com/conclusion.htm
It is the conclusion page of Fox's Regimental Losses. The essay addresses the very thing you are concerned about, accuracy of casualty rates, and from that, the number serving. Joe stated the same concerns in a humerous vein. I had a Great Grandfather and two great uncles in the 18th Georgia. The CWSS lists them enough different ways to be at least a squad on their own; and My GGF died in 1862! Stan