I tend to think of five-panel pages as the default for a general storytelling page, and then I add or subtract a panel or so as needed. I tend not to go over six panels on a page because things start to get kind of cramped and you have to be VERY careful about what you put on a six-plus panel page.

Of course, you generally want to mix it up! Depending on your storytelling needs and pacing requirements, you might have plenty of three- or four- panel pages. And splashes! And double-page spreads!