I thought I'd include a portion guide here.
Of course it all depends on how your cake is cut as to how many clean portions you'll get from your cake.
Fruit cakes are usually cut into 1x1 inch slices whilst sponge cakes are cut into 2x1 inch slices.
A good tip for cutting fruit cake is to use a serrated knife and to wipe the blade after each cut with a clean cloth. This will stop the knife dragging through the cake and will leave each portion clean and tidy.
Fruit cake Round Square
6" 22 27
7" 30 40
8" 40 54
9" 54 70
10" 68 90
11" 86 112
12" 100 134
Sponge cake Round Square
6" 11 14
7" 15 20
8" 20 27
9" 27 35
10" 34 45
11" 43 56
12" 50 67