When William Tecumseh Sherman undertook his March Through Georgia, he commanded three armies which totaled approximately 100,000 men. The Union Army had approximately 1,000,000 men under arms at the time of Appomattox, according to Samuel Eliot Morison.
In February of 1882, the same William Tecumseh Sherman reported the following to the Secretary of War.
The Army to-day consists of 23,785 enlisted men and about 2,000 commissioned officers. Of these (the following) are posted west of the Mississippi River--
All of the cavalry--ten regiments 6,627
All the Indian scouts 300
Twenty-four out of twenty-five regiments of infantry 10,916
First Regiment of Artillery 525
Batteries E and F, Second Artillery, and C, Third Artillery 161
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Total west of the Mississippi 18,529
East of the Mississippi River--
The Tenth Regiment of Infantry 350
Four regiments of artillery, less the three companies 1,885
Permanent recruiting parties 1,316
Engineer Battalion, Ordnance Department, and non-combatants 1,705
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Total east of the Mississippi 5,256
One further point of interest, the Secretary of War to whom General Sherman reported these figures was the Honorable R.T. Lincoln-Robert Tod Lincoln, the son of the late president under whom Sherman had also served.