The idea of performing Surya Namaskar 12 times is derived from a
Solar mythology, whereby the Sun ("as deity of light and warmth, on whose influence all organic life directly depends") has twelve names, as well as 12 horses (representing the 12 months of the year) which
pull the Sun’s chariot across the sky, passing through the 12 zodiacs
in the heaven. 

The twelve names of the sun are as follows:
 

These energize your personality, make it sensitive to the cosmic power and cure you of all blemishes, such as disease, despondency or lethargy. 

To start with, it would be wrong, and perhaps even harmful, if this exercise is not done very slowly and without any exertion. To do this exercise without exerting yourself is the most important thing. In brief, the whole point of Yoga is that you never hurry or force anything.