Do we know that Simm -> Gomez is the correct order? I could see them twisting it and having Gomez regenrate into Jacobi, maybe. Given how the order doesn't *really* matter for the Master
Wasnt the first Master to be introduced the very last of his regenerations? Theres 12 other unknown Master incarnations before Delgados version. If you were going to go all out on a multi-Master story you could do one where the reveal is that every character in the story besides the Doctor is The Master.
About the bolded: well technically we don't know.
The first Master we saw was Delgado. We don't know the number of his regeneration. After Delgado died (in real life), they retired the character for a number of years, and when we saw the Master again he was that crispy skeleton, and we were told the Master had run out of regenerations. So that means he had 13 lives, and we had only seen 1 (or 2, depending on whether crispy skeleton was still Delgado or a different regeneration). But we don't known where Delgado was on the timeline. He could have been 1, or 4, or 8, or whatever.
After crispy Master, he basically stole a couple of extra lives - Ainley and Roberts were both stolen bodies, not traditional regenerations.
Presumably, at some point the Master received a new set of regenerations from the Time Lords. As Jacobi, he fled from the Time War and forgot he was actually the Master. When he remembered who he was, he turned into Simm, which was the only traditional Master regeneration we've seen (and that means we've only seen the Master transform from one form into the next twice: crispy skeleton into Ainley and Jacobi into Simm).
We don't know yet how the transformation from Simm to Gomez happened.
So yeah, we've seen 7 bodies: 2 (or, again, 1, depending on your interpretation) from his first set of regenerations, 2 stolen bodies, and 3 from his second set of regenerations. So that means there are
at least 11/12 incarnations we've never seen. And we've only seen a direct transformation from one body into the next twice.
So there's definitely a lot of interesting stuff they could do, although I doubt how deep they want to go with that. I've said this earlier, but it's unlikely they'll get Roberts back given rights issues with the movie (and the relative obscurity of that version of the Master), and Delgado and Ainley are both no longer with us, although they'll probably use some archive footage for Delgado at least. But I'm guessing it'll mostly be Simm/Gomez and maybe some weird twisty version of that regeneration - which would be fun to see but not essential, the weird chaotic background of the Master is part of the fun. As opposed to the Doctor, his regenerations don't need to be seen or explained. Although I'm guessing they'll probably do it.
** All this is based on the tv show, not the added stuff in the audio's, comics, books etc. I can't imagine they'll use any of the extra incarnations who weren't part of the tv show.
*** I've mentioned the idea that crispy burned out skeleton Master might be the same as Delgado, and although it's never stated outright on screen, most fans (and I guess writers too) consider them two separate versions. If only because it seems only right to treat Delgado's work as standing on its own.