How many episodes will there be?
Both Part I and Part II had eight episodes, so eight is a safe guess.
Who in the cast is going to return?
The OA wouldn’t be the same without its titular character, Prairie Johnson, the dimension-hopping, NDE-surviving OA herself, Brit Marling. Like Season 1, Season 2 ended with her fate being somewhat up in the air after experiencing a head injury, but Marling is basically a given to return, as she's also the lead creator/writer on the series and the story is so integral to her character's fate.
While that final sequence may have called into question Prairie’s future, it does conclude with Dr. Hap certainly alive -- even if he is cheekily inhabiting Jason Isaacs (with his signature British accent!) in a version of our own dimension. His arc is so intwined with Prairie's, that it makes sense he’ll be back.
Kingsley Ben-Adir who played private investigator Karim Washington in Season 2 will also likely reprise his role, per recent interviews. And in the infamous Old Night scene, Washington's briefly referred to as the OA’s brother, which, of course, wasn’t absent of meaning -- this is The OA here. Ben-Adir told Vulture, "That’s a secret of the creators. I think that’s something for you guys to think about and find out later, so I’m not going to say anything about that," meaning there’s probably a lot in store for his character down the road.
But as for the rest of the characters, most of whom ended up as a sort of garden of water lilies at the end of last season, it/s hard to say. Homer, though, Emory Cohen’s fellow NDE-surviving character who Prairie has spent much of the series searching for, seems like a secured return. Since Season 2 ended with a very dramatic sequence of him finally coming into consciousness in the body of Dr. Roberts, it's implied he's probably got some more movements to do to be reunited with his space/time continuum traveling soulmate.