"Use Your Multiple Ad Units" Says Adsense
Inside Adsense have issued some routine advice on utlising multiple ad units. Essentially they suggest more ads increase your chances of relevancy to the reader. No surprises there really!
If you already have code for multiple ads but are seeing fewer units than you thought; Inside Adsense explain "this is because our multiple ad unit system works best for pages with highly targeted ads and therefore may only display the first few units (See how a first ad unit is defined)"