The capacity of a stock #5, i.e. w/o any spacers, is approx. 2.5 gal.
Let me know if you are looking for, in my opinion, the best drive fluid available. I have 10 NEW gal. available from when I had my Cigarette running #5's. Sold the boat and have the fluid on the shelf. It is not cheap, but I never had a drive failure using it and whenever they were torn down and inspected the gears and bearings always looked as if they were new.