As DaveW stated, there is no theoretical limit placed by the rules on how much channeling you may legally have. Any limit would simply be imposed by the card pool available to you. Assuming you have access to all of the cards currently printed, the highest you could currently get your Mage's Channeling up to is indeed +14 over your base (or 24 total). If mana is your goal though, you could theoretically be producing much more than that if you use Spawnpoints and Familiars.
From my calculations, a Warlord under ideal circumstances and card selections could be producing up to 35 mana per round between his own channeling, his spawnpoints, and his familiars. For much fewer spellpoints, a Wizard could produce 34 mana per round. I wouldn't really call any of these goals "practical", however, and we are now getting into the realm of strategy and tactics rather than rules. Per the rules, the answer is a very simple "there is no limit."