Excerpt from a November 1861 army circular issued under signature of J. P. Benjamin, Acting Secretary of War, C.S.A.:

10. A regiment is by law composed of ten companies, neither more nor less. A battalion is entitled to no other field officer than a major, unless the number of companies exceeds five.

However, as Danny and Doyle stated, the Confederate army often opted for expendience over rules, and many regiments had fewer than, or more than, the mandated 10 companies.