Step 1: Preferential treatment
Access the Preferences by clicking on the iPhoto menu at the top of the interface and then selecting, you guessed it, Preferences. This will bring up a new dialog box in the middle of your iPhoto screen.
Step 2: Click it
With a simple click you can turn on the Item Counts. This will then show how many photos are in each section on the left-hand pane of the main interface. Use this button at any time to toggle the stats on and off.