How do I delete multiple photos at one time in iphoto?

select them and hold down the command key and press delete - or right click and select delete "n" photos

LN

Go into your photos and click on a photo. Then hold the shift key down. While continuing to hold the shift key down, use the arrow keys to highlight any photos that are nearby

That's one of the ways to select multiple items - and they are the same throughout the OS.

Click to Select

Command-Click to add to a selection - particularly useful if the items are non-contiguous

If the items are contiguous: Click on the first item, hold the shift key and then click on the last. All the items in between will be selected

Or

Drag a rectangle across all the items with your mouse.

To delete things:

First move them to the iPhoto Trash

Then empty that. They then go to the System Trash. Empty that.