Ah, but did he even escape? We don't know, hasn't ever been explained.
But the way I see it, there are multiple Rassilons: the original Rassilon, whose body was preserved in the Tomb of Rassilon and eventually brought back to life during the Time War; and the copy in the Matrix, who became trapped in the DivU. Thinking about it, there might be something in "Zagreus" that indicates there's only one Rassilon - I recall some scene where Romana and Leela visit Rassilon's tomb.
Alternate theory, Rassilon was "resurrected" during the Time War by being plucked from the timelines at a point when he was still alive.
Basically, I don't think Rassilon ever got out of the DivU, but all I have is theories, no definitive facts.
"Only Human" is pretty great. tardis.wikia.com/wiki/Only_Human_(novel)
Ooooh, good question! Well... I'd count 18 "main" companions, companions who were at one time or another appearing regularly in Eight stories.
Stacy Townsend & Ssard: Radio Times Comics; Izzy Sinclair, Kroton, & Destrii: Doctor Who Magazine comics; Sam Jones, Fitz Kreiner, Compassion, Anji Kapoor, & Trix MacMillan: BBC Books; Charley Pollard, C'rizz, Lucie Miller, Tamsin Drew, Molly O'Sullivan, Liv Chenka, & Helen Sinclair: Big Finish audio; Josie Day; Titan Comics.
But there are more. 43 pages are in Tardis Wiki's "Eighth Doctor companions" category.
Grace Holloway and Chang Lee, from the movie. Miranda Dawkins, the Doctor's adopted daughter who he raised for over half a decade in the novel "Father Time". Gemma & Samson Griffin, who first appeared in a Big Finish audio set after they finished travelling with the Eighth Doctor and later appeared several shorties. Mary Shelley, who the Doctor travelled with in a Big Finish trilogy. Deborah Castle, who was good friends with the Doctor for a full decade in "Father Time". Nyssa, who he spent a few months with in the audio "A Heart on Both Sides".
The novel "The Year of Intelligent Tigers" features flashback to a time when the Doctor was travelling with a man named Lorenzo Smitt. "Intelligent Tigers" also has the Doctor in an adorable friendship with a really cute talking tiger named Bounce, who I personally consider to be a companion.
And there's Scarlette, the Eighth Doctor's wife in the book "The Adventuress of Henrietta Street".
Eight also has several individuals who were simply mentioned to be his companions. The Big Finish audio "Mary's Story" mentions a Rita and a Todd. The BBC novel "The Gallifrey Chronicles" mentions Delilah, Frank, Claudia, Jemima-Katy, & Nina. IMO the story behind Claudia is a bit interesting tardis.wikia.com/wiki/Claudia_(Father_Time)
The wiki's Eighth Doctor companion category lists a few people that I wouldn't consider companions because of how little time they spent with Eight: Susan Foreman, Fey Truscott-Sade, River Song, Maxwell Edison, & William Shakespeare.
So... it's a bit subjective, but I'd say: 35 or 36. And I'm almost certainly missing someone.