The number of regiments in the Confederate and Union Armies during the Civil War varied significantly over time, but neither side was limited to just two, three, seven, or fifteen regiments. At their peak, both armies had hundreds of regiments.
For accurate context, the Union Army had over 2,000 regiments at various points, while the Confederate Army had around 1,000 regiments. Therefore, none of the given options (a, b, c, or d) accurately reflect the number of regiments in either army during the Civil War.