For a regiment, MOST of the time it was sufficient to send a single company out as skirmishers. There were occasions when a brigade could, and did, use an entire regiment as skirmishers. Having said that, those latter situations are more of a case of time/place/unit situation rather than the norm.
In other words,……. "it all depends."
The skirmish line (ACW) was deployed in depth and controlled by bugle calls. A company would be sent out, with one platoon being kept as a reserve for the line, while the other platoon deployed.
Deployment was by groups of 4 men, referred to as "comrades in battle". the 4-men consisted of 2 adjoining files from the company's formation in line.
The groups of 4 would first take intervals from each other, either taking it from the center, the left or the right as the situation and terrain dictated. Once the 4-an groups had deployed, then the individual men took their intervals, at about 6-10 paces between the 2 files.
If ordered to advance,the entire line would go forward at a walk. When resistance was encountered the command to commence firing may be given. When that happens, the front rank men fire, then reload. When they have reloaded, the rear rank men will fire. Mind you, this is done across the entire front, BUT firing and reloading are done at the pace of the front rank man and his partner behind him, with each file firing at their own speed and independent from everyone else.
Advancing and retreating while firing was done in an "overwatch" manner (to use the modern term). As the front rank man fired, the rear rank man would advance past him, and beyond about 6-10 paces while he reloaded. When ready, the rear ranker would fire, and as HE reloaded, the front rank man would advance past and beyond him, etc etc etc.
Retreating while firing just reversed the procedure.