Update: With 50 hours to go until the competition starts, deck count is now at 24.

I know that at least one more deck is currently in testing, so we'll have 25 decks minimum. If I don't receive any more decks, then the fixture for the competition will look like this:
First stage, 5 groups of 5 decks each, first 3 of each group get promoted to next stage
Second stage: 3 groups of 5 decks each, best 3 of each group (except the third-placed team with the least points) get promoted
Afterwards: Quarterfinals, semifinals, final as one-on-one knockout stages.
If I receive more decks (but less than 7), then we'll have a competition of 32 decks. I'll add some experimental or themed decks of my own to get the count to 32 if necessary - the decks I add won't be high-performance decks, but shouldn't be pushovers either, in any case it's nicer to have a full field of competitors than giving some decks a bye. With 32 participants, the tournament will go through these stages:
First stage: 8 groups of 4 decks each, best 2 of each group get promoted
Second stage: 4 groups of 4 decks each, best 2 of each group get promoted
Afterwards: Quarterfinals, semifinals, final as one-on-one knockout stages.
Matches will be played as "Best of 7" from the quarterfinals onwards and "fixed N games" before, I'm currently contemplating on the exact value for N (I want draws to be a possibility in the group stage). Before the competition starts, a large number of fast-paced "training matches" between random competition decks will be played so that the AI can get accustomed to the decks.