UPDATE: To email multiple photos with iOS 7 follow these instructions:
1. Open the Photos app -> Camera Roll or select an Album
2. Tap Select in the upper right corner
3. Choose up to 5 photos for emailing. Selected photos display a blue check mark
4. Tap the share button in the lower left corner
5. Choose Mail from the pop-up menu to email your photos
Unfortunately, for now, you can't.
The current version of firmware on the Apple iPhone provides no method of emailing more than one photo at a time.