Enough for your expected passage time plus two weeks.

Sometimes the passage takes longer than you expect.  Maybe the wind is light, or you sail too far north, or you have a gear failure that will slow you down (no mast, rudder, etc).  The extra two weeks' worth of food should not be frozen and need not be real good.  Heck, a case of ramen and a case of Dinty Moore will cover you.

Don't cut this too fine.  You will be mocked if you run out of food.