Re: The Amber Fight
If memory serves correct, it's just Saber and Kohaku. I've got a stage 51 save currently, so give me a few days and I'll reconfirm.
Edit: (This shit's gonna have spoilers)
Ah, I forgot that Arc joins up with Amber and Lily. I think you play with everyone besides those three during the final climactic battle. There are multiple battles leading up to that point though, where you are considerably more restricted.
Unplayable characters across all battles: Kohaku/Amber, Saber/Lily, Arcueid/Phantasmoon/Eclipse (and Mech Hisui?*)
Playable characters are as follows:
Battle 1: Rin/Ruby, Shirou, Sakura, Rider
Battle 2: Rin/Ruby, Shirou, Sakura, Rider, Karen, Lancer, Archer, Ciel, Hisui (and Mech Hisui?*)
Battle 3: Ryougi, Touko, Takumi, Haruna, Satsuki, Sion, Len, Bazett, Natsuki
Battle 4: All of the above + Shiki and Akiha (and Mech Hisui?*)
*Note: I don't have Mech Hisui unlocked in my file, and you fight against her in the final battle; however, there is exactly one extra placement space that could only possibly be filled by an allied Mech Hisui in both Battle 2 and Battle 4. It seems that she is able to join you, but I can't be certain without confirmation from someone else.
Anyway, assuming you have both Sacchin and Mech unlocked, that makes a total of 20-21 playable characters in that absurdly large final battle. Have fun~