I make my cakes 4 layer cakes. My 9" molds are only 2" high so I torte two in half and get a 4 layer cake and that's the size I sell the most. I thought this was the norm however, lately I've been seeing pictures of a lot of 3 layer cakes and I'm also surprised at how thick each layer is. Are people doing this by adding extra batter and charging more? Or baking in 3 separate pans? I'm curious.

Also I recently saw some episode of Cake Boss where Buddy was tearing someone a new one for having made a 4 layer cake instead of 3. I think it was a chocolate something recipe and it turns out the cake started bulging and sagging under its own weight and Buddy was furious that they should have known that recipe couldn't handle the weight of 4 layers. Which obviously makes a lot of sense and got me thinking that could be the reason I occasionally deal with cakes starting to sag come delivery time. So besides the obvious pound cakes and angel food cakes, is there a rule of thumb to go by when it comes to cakes, fillings and choosing to make 3 or 4 layers? 

Do whatever you want.  If you are successfully (and profitably) selling 4 layer cakes--keep doing that.  It's like asking somebody what kind of car you should drive--the answer:  whatever you want.

I do 4 layers  always, doesn't matter what recipe this is. Each layer is 3/4 inch tall, and put a buttercream dam only on first layer to prevent bulging.

I make all my cakes around 4" high.  I have 9" pans that are 11/2" so my batter makes enough for all three.  This makes a 3 layer cake.  My other pans are 2" so I make 2 - 2" layers so I split each in half.  This will give me a 4 layer cake.  I was just in a party this weekend where I brought my cake and someone commented to me that what they liked was that I had provided lots of cake.  I guess they are used to the store bought which is usually just 2 small layers.  So like Apti said 3 or 4 makes no difference I just work around my pans and my batter.

I make all my cakes around 4" high.  I have 9" pans that are 11/2" so my batter makes enough for all three.  This makes a 3 layer cake.  My other pans are 2" so I make 2 - 2" layers so I split each in half.  This will give me a 4 layer cake.  I was just in a party this weekend where I brought my cake and someone commented to me that what they liked was that I had provided lots of cake.  I guess they are used to the store bought which is usually just 2 small layers.  So like Apti said 3 or 4 makes no difference I just work around my pans and my batter.