Lizardmen Army Book, p.43:
Lizardmen FAQ, p.3:The Focused Rumination
When the Slann casts a spell, a 'free' power dice is added to the attempt. This ... can cause the Mage-Priest to roll more dice than normally allowed.
BRB, p.30:Q. Does the ‘free’ power dice gained from Focused Rumination
count against the power limit? (p43)
The FAQ is specifically about the power limit, ie., the total number of dice a player can have in his casting pool. If you roll double-six on the Winds Of Magic, you can't use The Focused Rumination to generate an extra dice as you would exceed your power limit of twelve dice. However, the rule for The Focused Rumination specifically says you can use the extra dice to roll more dice than normally allowed, which is a reference to the six-dice limit mentioned in BRB p.32.POWER LIMIT
There is a finite limit on the amount of power a Wizard can control. The casting player's power pool can never exceed 12 dice at any point in the phase - any dice generated beyond this number are lost.
@Jesse Newton is 100% correct.