2 movies would work best. One would be "OK."
__You COULD do Venom and Carnage in one (provided Spidey had the sym suit in the previous movie). You'd just do Venom's orgin, slap him in prison, Kasady's origin (first arc), Spidey/Venom team-up for the big finale. Yeh, it'd be shaved to the bone but might actually make for a nice, tight action film. I personally wouldn't feel short-changed because I got all the back-story and depth in the comics. What I want from the silver screen is the visual and kinetic presence of the symbiotes and a faithful charactorization. An abridged version of the saga is fine.