If we were to go by total domestic box office (not adjusted for inflation) for all the films that the particular star appeared in, it’s not Stallone:
Daniel Radcliffe has appeared in the eight films of the Harry Potter series which have a total domestic box office of $2.39 billion. The total of the six films of the Rocky series is $566 million. It appears that the total box office of the four Rambo films and the two The Expendables films were each less than that, so Radcliffe has made more from his eight series films than Stallone has made from his twelve series films. The other obvious actor is Harrison Ford. He co-starred in three Star Wars films and starred in four Indiana Jones films.
It’s true that Radcliffe hasn’t appeared in any other series yet, but he’s presumably got a long career ahead of him. Any other series he gets cast in could easily put him ahead of Stallone for total series films made. If sheer number is what you want, I suspect that there were people starring in film series back in the 1920’s to 1950’s that beat either of them. Also, I suspect that are people starring in films in India that have more series films than anyone mentioned so far.