One and a half.
Arguments for one:
1. In poetry it usually scans as one:
Sol thro' white curtains shot a tim'rous ray,
And op'd those eyes that must eclipse the day;
Now lap-dogs give themselves the rousing shake,
And sleepless lovers, just at twelve, awake:
Thrice rung the bell, the slipper knock'd the ground,
And the press'd watch return'd a silver sound.
-- Alexander Pope, Rape of the Lock, Canto 1, ll. 13-18.
2. Can "twelve" by hyphenated at the end of the line? "Twe-lve" and "twel-ve" both look dubious. None of the dictionaries I've checked give a hyphenation point for "twelve".
Arguments for two:
1. If you can hear it, it's there.