I am not a doctor, but from what I know, one needs to take metronidazole twice a day. 7 days should be fine, which is precisely what 14 tablets translate to (if taken twice a day).
However, the best option is to call the doctor and double-check. Taking inappropriate dosage of antibiotics may prove to be harmful.
The other thing to take note of is that, one ought not take antibiotics too frequently if that can be avoided, and metronidazole is an antibiotic. So in case BV recurs, the situation may become tricky.
So my recommendation would be, while you take antibiotics, also work in parallel to fix the vaginal acidity and bring it back to normal levels. The real cause of bacterial vaginosis is imbalance in vaginal acidity. This leads to growth of unhealthy bacteria inside the vagina. And that translates to occurrence of BV.
Metronidazole, being an antibiotic, will kill the bacteria and keep the vagina free from the unhealthy bacteria for 3-4 weeks, as long as the effects of the antibiotic lasts. After that, if the vaginal acidity is not healed (on its own, or by your active efforts), the bad bacteria will come back and BV will recur. You will have to take another round of antibiotics in that case (and it will continue).
So a great idea is to try and restore (fix) the vaginal acidity to the level possible. Substances such as apple cidar vinegar, cranberry juice, yogurt and tea tree oil will help.
If the vaginal acidity is back to normal levels, the harmful bacteria will die irrespective of the antibiotics, as the vagina will no longer remain a healthy place for such bacteria to grow and breed. That will cure BV and prevent it from recurring.