This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The subclavians, carotids, and vertebrals, have been observed to arise separately from the arch, each vertebral being between the subclavian and carotid of its own side.
Hence it appears, that omitting the coronary arteries, two is the smallest number, and six the greatest number of primary branches arising from the arch : these extremes are much less common than the intermediate numbers.
Sometimes varieties with regard to the coronary arteries have been observed; there may be but one; again, there may be three, or even four of the vessels found. These deviations are, however, rare.