classical
Since I frequently got composer's block in college, I'd make it a point to ask teachers what methods they used to kick it. One suggestion is to not write the whole piece from beginning to finish. Write the ending first, or save the intro until you've got something to introduce. Have a bunch of ideas you want to put in a piece but don't cement them in order; see if one works better before or after another. It's really easy to get stuck in a mindset of going from point A to point B, but you have to remember that's what the outcome will be. Nobody will know if you wrote the thing backwards, and even if you told anyone, some wouldn't care, and some might like the piece even more.
Also, it's usually helpful to plan out the entire thing first, and then start working, changing things as you go that don't suit your liking. Most of my composer's block on my current mix was due to the fact that I was writing it as I went, so when I finished a section I was like "Alright...now what," and then have to figure it out, rather than having it waiting for me.
BUT, to answer the topic at hand, however many sections you damn well please! I would call slight changes A* & B* (* = prime, so A-prime, B-prime, Metroid-prime, etc.) so that you know it's the same section occuring with a slight change rather than having sections A-E with B through E all being slight variations of A. That way you can organize a bit better with ideas and not go too extreme with labeling; however, that's just how I would do it. As Ecto said, it varies from person to person on the method that you choose to use.