How many elements are too many for one screen of a mobile (iOS/Android) application?

Without scrolling, what would be the maximum amount of centered buttons I could arrange on a vertical axis before it became too cluttered? Interface Builder gives suggested distances between elements, but should there be whitespace on the top and bottom as well?

Specifically, I have a simple utility app with a single screen. On this screen is an ad, two lines of text, two function buttons, and a footer image. I am thinking of adding a third feature. Although this is the specific instance, I’m looking for more general guidelines. When is it too much? When does it all become visual clutter?