How many sticks are in 1 cup of butter?

Two (4-ounce) sticks of butter equal 1 cup.

For more information about using butter in recipes, see the following Ask The Expert articles:
Can I use butter in place of shortening?

Can you substitute oil for butter?

Can you freeze butter?

Can I use melted butter instead of softened for baking?

Why do many recipes call for using unsalted butter?