This is an interesting question. During the Vietnam War, soldiers would have to be twice wounded, or wounded severely, before they would be discharged. Not that Taylor is wounded once in the initial fight and once in the final fight. Only at the point is Taylor shipped home.

During Vietnam, each brigade or battalion was made up of companies, which were made up of platoons. Each platoon was made up of squads. The movie is largely inaccurate in regards to ranks. In real life, the platoon sergeant would be a Sergeant First Class and the squad leaders would be Staff Sergeants. Barnes was a Staff Sergeant and also the Platoon Sergeant.

Regardless, you had Barnes, Elias, Warren, and Red. Elias, Warren, and Red each commanded a squad. Barnes directed each of these three sergeants and conferred with Lt. Wolfe. This is accurate because a lieutenant would depend on his platoon sergeant for guidance, though the platoon sergeant would generally show greater respect for the rank.

The film follows a single platoon which is part of the company commanded by Captain Harris. Each company would have 3-4 platoons: Alpha, Beta, Charlie, and Delta. Generally, Delta Platoon would be a mortar platoon. Though not seen in the film, it is presumed that the frantic radio call before the final battle is from one of the company's platoon, who are all killed or captured. The final battle then involves Wolfe's Platoon as well as perhaps others.

There are still possibly other functioning platoons in the company. Generally, unless the battalion or brigade was destroyed, new recruits would be brought in to fill the ranks. For organizational purposes, the company would retain the same name.