Your really need a calibrated corrosion cell to monitor how MUCH/many zinc anodes to use, especially if you are experiencing a high replacement rate, etc.. Otherwise, its 'just a guess'.
Over-zinc the underwater gear, and barnacles and other hard growth fouling 'will' become a problem. Lose an anode and there will be 'less' hard growth - probably the migrating ions 'disturbs' and 'annoys' the barnacles, etc.

example: http://www.boatzincs.com/corrosion-reference-electrode-specs.html