Officially a Regular US Army Regiment had 3 Battalions of 8 Companies each. 2 Battalions were supposed to be in the field w/ the 3rd recruiting & training.
This differed from the Militia & Volunteers which had 10 Companies which might be divided, at most, into 2 Battalions.
Officially in May of 61 Brigades were supposeds to include 4 Infantry Regiments w/ an
Arty Battery. Divisions 3 or more Brigades. In 62 the Corps was created. But as you all know that was a guideline at best and it was pretty rare for a Regiment to take the field at anything like full strength. IIRC Chancellorsville had Regiments on both sides averaging about 450 men.