When planning a Civil War reenactment, both sides use these unit designations. Scott H. Harris, director of the New Market Battlefield State Historical Park and the Shenandoah Valley Tourist Information Center, shared the following numbers although they are official strengths, almost never attained in real conditions.
A company is the smallest organizational unit, usually around 100 men. A regiment consists of ten companies, 1,000 men. A brigade is four regiments, 4,000 men; a division is three brigades, 12,000 men; a corps is two to four divisions, 24,000 to 48,000 men; and an army is two or more corps, anywhere from 50,000 to 100,000 men.
This breakdown is for infantry; cavalry and artillery units were organized differently. Cavalry also used companies, regiments, and brigades. Artillery was divided into batteries of generally fours guns (Confederate) and six guns (Union), though there were exceptions. Larger cavalry and artillery groupings were usually assigned to particular infantry brigades, divisions, or corps.
Reenacting today is composed of many rather small units that often portray a single company or regiment or battery in the case of artillery, says Harris. While they may participate in small living history programs or encampments by themselves, they usually affiliate with a larger organizational structure for reenactments (brigades, divisions, corps, or armies). On the Confederate side, among the largest of these are the Army of Northern Virginia, Longstreet’s Corps, and the 1st Legion, Provisional Army of the Confederate States (PACS). For the Union, the big ones are the United States Volunteers (USV), Birney’s Division, Vincent’s Brigade, and the First Federal Division. Most of these umbrella organizations take their names from their Civil War counterparts. Each has a commanding colonel or general, with his own staff that interacts with the smaller independent units.
All of the ranks above private, including non-commissioned officers like corporals and sergeants to officers like lieutenants, captains, majors, colonels, and generals, are typically elected by their comrades to those ranks. This was actually a fairly common practice in the real Confederate army, and to a lesser extent in the Union volunteer regiments.