Probably about 3-4 each side of the hull, 1 on the prop, and 1 each side of the rudder! get ready to dispose of lots of beer tokens: /forums/images/graemlins/smile.gif
Would be very surprised if the boat is not already fitted with anodes, although if it is in fresh water they would be magnesium not zinc. If you are not getting problems with galvanic corrosion on props and other bronze fittings (if you have them) there may not be a need for more anodes. As suggested, find out what you have and if you need advice contact Duffs.