This is not about how many, but how related your keyword from your content. I think there is no rule. in word count this is about the density of your keyword within your site.

Are you SEO???

number of pages only becomes an issue in more competitive keywords, it usually makes sense that a site with more content about something should rank higher than a site with one page about it.

but its more than just more pages, its also about internal linking structures between the different sub groups of content.

say your main theme is "widgets"

a one page site just about widgets, is generally less likely to do less well that a 20 page site with widgets as the main theme and 19 sub pages on big widgets, small widgets, blue widgets, eco widgets etc etc all interlinked thematically and all backing up the main page.

ya get me?

if you want to know how to easily work out what your sub theming should be about, read this article

Advanced SEO Web Site Design - Structure Site & Navigation for Superior Google Rankings