There is only one Prism every generation. He / She senses the balance of the world's magic. Can balance the magic, and can split the light within his / herself. His / Her role is largely ceremonial and religious, not political. Except for balancing the world's magic so that color wights and catastrophes don't result. The current Prism is Gavin Guile.
Prisms typically die (or start to lose their colors) after their 7th, 14th, or 21st year of serving.