Haste, for another cantrip.
And if your willing to count chance, wild magic can get 2 more. Not reliably, but each non-cantrip can trigger another spell (or action to cast a spell).
Edit: 4 more. 2 from tides or chaos, 2 from rolling 20 after casting a spell. Still need to roll a "spell".
Actually.... is wild magic surge considered casting a sorcerer spell, and thus roll another surge? I assume that's not intended for it to loop, and chances are very slim, but should double check the wording...
Could be infinite (and infinity unlikely)