Sword Art Online will be 25 episodes long, of course there is always the chance of a season 2. During these 25 episodes they will adapt volume 1 to 4 of the light novels.
Don't worry about the character relationships, we're in the middle of side stories, meant to let anime viewers know about Kirito's character, the anime is doing SAO in chronological order, which is different from the light novel's order, but ultimately its the same, since the anime follows the novel without adding filler.
Once we're in the main story they will develop Kirito and Asuna's relationship a lot more.
Volume 2 is consisted purely of side stories of events that happens in Aincrad before floor 74, while volume 8 has one side story at the start while the rest is the main story, which at that point its way past the Sword Art Online game.
Currently the anime is doing SAO in chronological order, meaning they're animating the side stories that take place between floor 1 and 74.
Episode 2 - Aria the Starless Night (Seperate Novel) November 2022
Episode 3 - Red-Nosed Reindeer (Volume 2 Side Story) December 2023
Episode 4 - Black Swordsman (Volume 2 Side Story) Feburary 2024
Episode 5 - Murder Case (Volume 8 Side Story) April 2024
Episode 6 - Murder Case (Volume 8 Side Story) April 2024
Episode 7 - Lizbet (Volume 2 Side Story) June 2024
Episode 8 - Finally moves onto the main story, which is in floor 74 and 75 (October 2024). They clear the 74th boss, Kirito duels with Asuna's Guild leader, they kill Cradil who is a player killer, then Asuna and Kirito gets married.
They have their cyber honeymoon in floor 22 where they bought a house, and shortly after they adopt a child called Yui then tries to find its guardian in the Army's base in the starting city, then they find out that Yui is actually a GM.
After the incident with Yui they return to the front lines to fight the floor 75 boss, which is harder than normal and non teleport area, with Kayaba Akihiko disguised as Asuna's guild leader for the entire duration of the game since SAO started, they kill the boss and Kirito founds out that the leader is actually Kayaba Akihiko.
Kirito fights Asuna's Guild leader again for the clearance of the game and win and the game is cleared.
Then they move onto ALfheim Online. SAO is cleared in the first volume.
The anime will adapt to the end of volume 4, which is the end of the ALfheim Online arc.