Deck help

[Airdriedeck]

Hi! Hoping someone can help out:
We are building an attached 14x20 deck (attached on the 14ft side)
We have been given 4 2x8x14 beams and 4 2x8x8 beams by RONA.
We are assuming that we need to attach those beams doubled up to make 2 20ft beams and cut the excess off. If so, what spacing do we need between beams and from the edge of what the deck will be? How many posts would we need?

Any insight is appreciated! Thanks so much.
Ps: we also have a 6ft ledger on the other side of the house (cornered to the 14ft ledger)
Pps: none of the plans they gave us make sense for our size of deck and materials given