50% of those have to be cameos, right? They're bringing back Stephen McFeely as a writer - he worked with the Russos on Infinity War/Endgame - so I'm convinced there'll be a good story.
So far, there are 27 stars confirmed to be in the movie, and 31 more rumored. If they're all in there, then yes. I have to assume the overwhelming majority will be a very brief cameo. I'm just speculating here, but I believe the focus will be on Doom, the FF, Sam, Steve, Peggy, Namor, Loki, Sentry, Captain Marvel, Photon and Starlord. Everyone else will cameo and MAYBE get two or three lines.
I gotta believe Spidey, Wolverine, Deadpool, Wanda and Vision will be in Secret Wars.
Daredevil? Luke Cage? Jessica Jones? Iron Fist? Punisher? Agents of SHIELD? Ghost Rider? They're probably a no. And man, I forgot about the Eternals. I think even the Eternals forgot about the Eternals.
So yeah. That's a LOT. Of course, the way movies are made these days, most of them probably shot their scenes separately. I doubt if everyone was on set the same day very often, if at all.