I have from as little as 3 (Apexplorers) to 9 (zombie hoard) so I'd say depending on what you are going for you can have a lot of versatility in your shelves.
The more figures you put in, the less interesting the display. If you stuff 8 - 10 figures into one shelf, none of them stand out. No one is special. I find no more than three is the best number to keep everyone looking good.
Thanks, you can give Cocoboloboy credit for that one... its a corner shelf he made for me. Guy does awesome work
And Apexplorers, if the ?? referred to what are they is a series from Winson Ma and produced by Hot Toys. There is a vague backstory for them, but I just love the look of the figures