If you google "5x7 font" you can get an idea of what the stuff displayed on them at 'nominal' resolution of one character per panel. You could turn them sideways to use a shorter font (5x3) and get more characters, or use them in a pictoral mode as just a graphic display of whatever size. I agree that 4 would be the baseline for a lot of projects, but 6-9 could give you a fair bit more flexibility in what you could display - scrolling text is a bit harder to get away with when each dot is physically switched and clicks.
It's the kind of thing where the project probably comes from the stock you have, though, so there are likely some creative ways to use however many to get what you want done.