Mord, I'm not clear by what you mean by zoomed? Are we talking about being in full-screen mode and then hitting the Z key? How do I measure what is acceptable?
Edit: OK, I think the answer is 2. No matter how many images I select when I'm in full screen and hit Z I am left with 2 full images zoomed to 100%. I even tried plugging into my TV for 1920x1080 display to see if it was limited by the screen resolution. Machine is a first generation 2.0Ghz MBP (15") with 2GB of RAM.