It really depends upon the size of your cupcakes as to how many you can fit on there. Some people have smaller "large" size cupcakes than others. If you don't use big swirls of icing on top that extend past the cupcake's width, you can cram them in closer together to add even more.
You can see the layout more clearly on these pics. (The blueberry cupcake display didn't require that we use the missing bottom tier, but the top ones are full. In the aqua cupcake display we only used 48 on the middle tier....but you get the idea for positioning!
Usually, it can fit comfortably.....
4 on the top tier
12 on the 2nd tier
48 - 56 on the 3rd tier
72 - 80 on the bottom tier