Platoon wasn't always called Platoon.

The very first version of what became Platoon was called Break, and it was a whole different type of story (much more mystical, more confusing, etc.). When Stone sat down to deal with Vietnam again (in the mid-seventies), he took some of the characters from Break and called the new film The Platoon. By the time he sat down to revise the script around 1984ish, he changed the title again to just Platoon.

So, why change the title so many times? Or rather, why just call the movie Platoon?

Well, for one thing the movie is about a platoon—an army unit usually comprised of about 40 guys. Okay, but the film just follows one specific platoon, right, so why not keep the title The Platoon?

Even though the film follows just one platoon, by making the title a little generic so that it doesn't name any specific platoon (as in "the platoon with Elias, Barnes, Taylor, Bunny," etc.), Stone was able to suggest that the events that happen in the film could have happened, and did in fact happen, to many platoons. The platoon we see is the vehicle for the film's themes, yes, but the title makes it clear that the experiences in the film were typical for any and every platoon.