How many layers should a drawing have?

Create independent layer for each element in your scene such as foundations,walls,columns , beams , doors , windows , electric devices and so on 

It depends on your work and what you want to draw

It depends on your work and requirement of element. Create independent layer for each element in your drawing i.e. foundations, walls, columns , beams , doors , windows, electrical, HVAC and so many more But be careful to create un necessary Layers because it may cause to make a heavy size of your drawing file. So when you complete your drawing should delete un necessary layers and purge the file to reduce the file size.

Unlimited.

Better to be as min as possible and grouped as per the project / requirement.

It depends on the purpose of drawing.There is no limit of layers.

It depends what kind of drawing we are working in. There should be sufficient layers created for simplying the drawing. Also, once you finish working with it, remove unwanted layers and Purge.

there is no limit of layers in auto cad, for each and every single element you can give a separate layer,this will make your work very easy.  

depending on what is the drawing

is it a building ? or is it mechanical  component? or a big assembly

it depends on the purpose of the drawing

It depends on the nature of your drawing and its elements. Layers are meant to provide convenience during work and time saving, Of course not to puzzle yourself or others, so only make layers which can be helpful during work and later on to review or edit.  if you have made some unwanted layers and you find it hard to identify use PURGE command after finishing your drawing. It will identify, automatically, and ask you to keep or delete them as well as any other entities with out reference.

there is no limit of layers . and how many elements in drawing .