How did sides in the american civil war meet demand for large amounts of manpower?

The was a great surge of patriotic volunteers willing to fight for the Union and Confederacy. Both sides inspired many volunteers to join their ranks by invoking patriotism and appealing to a sense of duty (which ultimately worked). There was no general draft or conscription until much later in the war when both sides began lacking soldiers.

So my final answer would be Volunteers!

Hope this helps! :)