What type of supports do I need for a 12'x20' deck?
Temporary post removal
If the beam members don't all join over one post, you can probably temporarily remove the two interior posts while you do your concrete work. Assuming adequate lap (with no more than one butt joint over each post) and adequate cross-fastening (say three screws every 18&quot;), it'll hold the empty deck for a span of 18'. If you see substantial pressure or movement when you remove the first post, consider temporary posts to the ground outside the concrete pour area.
You'll want to block off the entrance to the deck to make sure no one enters it during this time.
Permanent post removal
Generally speaking, the footings that would've been built in 2013 should be able to carry half the deck and its contents. Should. Without knowing what they are, we're speculating. 6x6 posts are also probably adequate, depending on specific local code. That leaves the question of the beam.
You'll need steel (or an outdoor-rated engineered wood beam), and you'll need an engineer to sign off on a size. Other than the challenge of moving such a beastly beam, it's more or less a drop-in situation. Ask your inspection office what type of fastening to the posts and joists they'd expect.
On the subject of beam connectors
I've never heard of a deck falling due to post or beam displacement, though I can imagine it happening in a severe wind situation or if someone hits one with a tractor. Having built dozens of decks through the 80s and 90s and beyond, we never used structural steel for that. Just toenailed screws. If you want to add something, go ahead, but I wouldn't worry about it.