Starting at the top:
5 rows of 3
This will give you 24 cupcakes
The top three and the bottom 2 rows will fit tightly together and be easier to ice,
leaving larger gaps in the middle section that will probably require more icing,
just try to place them as close together as you can.
Here is a video http://www.youtube.com/watch?v=EaZS1xjz--4
Just try to get a straight line on the edges.
Hope this helps, good-luck