For a deck that extends 20 ft from the ledger you would need 2 beams no matter what using dimensional lumber. Which means from what you are saying or what Im understanding you are about 8 pc 2x8x8 and 2 pc 2x8x14 short of building the deck. (using 16o/c joists). I assume you would be using 5/4x6 materials for decking.
For this you need at least 4 piles and posts and depending on the amount of weight you intend to put on the deck but I would normally use 3 posts per beam.
As mentioned before I think you need to get some professional help or at least have a pro draw it out for you with every detail so you dont cost yourself more money. Details that would also include doing the ledger and flashing properly so you dont rot out your house exterior prematurely or leave an issue for the next owner.
Just my 2 pennies...
Also, if you have a plan of some kind from rona maybe a pic would help, but in my experience they never account for double joists at the ends for railing or "picture framing" the decking nor the mid span blocking required. Which is why understanding all the details and having your own material list is always better then guessing how someone else might put it together ( properly or not)
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