On a standard weekend I will take 4 weddings. Why 4? Because through experience that is about the maximum amount of weddings I can comfortably take without feeling like I'm having to substitute quality for quantity. I want to be able I can give each tier the attention and detail it deserves.
However, this number will also vary based on the cakes themselves. If I have several very large cakes or cakes with a lot of detail, such as sugar flowers or fondant then I will often limit those weekends to 3 weddings because those cakes will take longer to decorate. Or, on the flip side if all my weddings are small with less time consuming designs then I can sometimes take on 5 weddings comfortably.
Don't feel uncomfortable asking your bakers about this. It's always best to get a good understanding of how your baker runs her kitchen!