My personal recommendation is to visit three wineries in one day, maybe four if you are militant about keeping to a timed itinerary. However, the number of wineries isn't strictly about logistics and timing. Consider whether you will actually be drinking the wines you are tasting, and if so, how much wine that actually is. Even though a "taste" is not a full glass of wine, it adds up. Additionally, you may be tempted to schedule more tastings if the tasting rooms all located within walking distance of one another, which is the case in towns like Los Olivos and Solvang. However, tasting 5-7 wines in each tasting room means your palate is going to be overstimulated. My personal recommendation: better to take it slow and absorb the moment!