Short answers, pulled from the story.
The Army of Northern Virginia officially replaced the Army of the Potomac on the 22nd of October 1861. This unit had been organized as the Army of the Potomac on the 20th of June 1861 under General P. G. T. Beauregard.
Robert E. Lee assumed command of the Army of Northern Virginia on the 1st of June 1862 following the wounding of Joseph E. Johnston at Seven Pines. Gustavus W. Smith briefly held leadership before Jefferson Davis ordered Lee into the position.
The Army of Northern Virginia numbered fewer than 40,000 men at the Battle of Antietam fought on the 17th of September 1862. The army lost approximately 30,000 troops within three months of Robert E. Lee taking command out of roughly 92,000 men prior to that period.
On the 31st of January 1865, 69,659 soldiers in the Army of Northern Virginia were fit for battle but over 4,500 had no rifles. This force included diverse regiments from across Confederate states including Texas, Georgia, Arkansas, and Missouri.
The Army of Northern Virginia surrendered to Union forces on the 9th of April 1865 at Appomattox Court House. General Robert E. Lee signed surrender papers to Ulysses S. Grant effectively ending the Civil War.