13.4v up to about 14.4v is what you should be seeing.
12.9v is still enough to keep the battery charging but suggests you may have a problem unless you took the reading when the engine was idling. Anything less than about 2500rpm will give a low reading.
As long as the voltage is between 12.5v and 14.5v you should be fine, any less is going to leave you with a flat battery and any more is likely to fry your battery and everything else...