I'd like to see it as a trilogy personally. There may be more later, but Toby McGuire has already said he doesn't want to do a 4th, so if they are getting a new actor, I think it would be best to wait a while and do a different storyline instead of picking up directly after 3. Kind of how "Batman Begins" feel much different than the other Batman movies. Spiderman should be the same. They should just do a few trilogies.
I could see it becoming a series, but just as long as they don't "James Bond" it. We don't need 8 people playing spiderman.
I believe I heard somewhere in some creator's commentary or something that they planned a tiny bit larger than 3. Maybe 4 or 5. But that's only planning, so nobody really knows if they'll actually make them. Either way, Can't wait for Spidey 3. Not to mention, I hope they make the movie game as good as last time. Especially the fight in the grocery store. Gotta love that. XD