As with the power pool, the number of dice in the dispel pool can never exceed twelve, regardless of how those dice are generated. Any excess is lost.
For example: It's the start of the Magic phase, and the casting player rolls to generate the Winds of Magic. He rolls his 2D6 and scores a 3 and a 5. The casting player therefore adds a total of 8 (3+5) power dice to his pool, whilst the dispelling player adds 5 dispel dice to his pool. Furthermore, both players can have their Wizards attempt to channel. The casting player has 3 Wizards, so can roll 3 dice in his channelling attempts. Alas, he doesn't roll any 6s, so gains no extra dice. The dispelling player has only 1 Wizard, so rolls a single dice, but gets a 6, increasing his dispel pool by 1 to an impressive 6 dice!