My idea of an "authority" site is one that covers the full breadth of its topic with quality, expertly written content.
So my answer to your question is, "as many pages as it takes to cover every aspect of your site's theme."
Use a variety of keyword tools to see what people are searching for that's related to your topic and create content around those queries.
As Homebizipedia noted, however, if you want any of those pages to rank you've gotta build links to them.
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