The Union army represents all 3 Union sub-armies together. The Army of the Potomac, the Army of the Cumberland and the Army of Maryland. It started off with 1,100,000 soldiers at the beginning of the Take Command War. However, it has slimmed down by some nearly 18,000 casualties.
The army was usually kept at an 800,000-size until they called for 300,000 more due to the uprising war.