Last year I had the pleasure of working on a deck project that involved a face-lift plus a structural repair to the main supporting beam.
The deck is an elevated second-story deck that sits over a 2-car parking area which required a 20 foot clear span of the main beam. The picture above shows some fancy engineering of a laminated beam made up of 2 x 12's and plywood spanning the 20 feet. It will not sag under it's own weight and will hold up nicely when your party of 12 sits happily and safely above!
The deck in the distance shows a 20 foot span sagging under it's own weight...evident in the slightly curved appearance to the bottom of the deck. Imagine what a party of 12 would do to it? When designing the structure of a deck, there are specific building codes to follow with beam and joist spans. Things are very specific in the types and sizes of beams to use according to the distance you wish to span from post to post.
You want to make sure your spans follow those guidelines for safety reasons... saving money or trying to maximize space under a deck by using less posts with the wrong type of beam could result in structural failure. The above picture shows what you get with the proper guidance of a knowledgeable contractor working on your job. The deck carpenter for this project was Tony Fredericks of The Woodcrafters (416-420-3999)