Since there isn't a minimum number, I don't think they look for a specific number. And they also don't ask a specific number (though they may ask how long the time frame you shadowed was).
With shadowing, clubs and pretty much your whole application, think quality over quantity. Even if you're only in two clubs, but you're the president of both, or extremely involved with both, it speaks larger volumes than someone who has 20 clubs, but isn't actively participating in any of them.