Thanks, folks - especially tomos, interesting suggestions.
I could have been clearer. What prompted the query was a desire to decide on my personal preferred ISO rating, and I wanted enough images visible at once, and as big as possible, to do that; and also later to compare compositions. I wasn't looking for a duplicate detector (yet), and didn't want to buy Lightroom if I could help it; it's enough trouble to stop AdobeARM running all the time

I agree that four images is adequate, though I'd have liked six if possible. Of course, then they get smaller. I see the well-liked
BreezeBrowser Pro only gives 2, 3 or 4. It looks like ExifPro Image viewer offers a large subset of the features of BreezeBrowser, at a much lower cost