1/2 syringe enough to cover gummy smile and uneven upper lip?

Botox will help with the gummy smile but not correct unevenness. If you are concerned with unevenness opt for a dermal filler as well.  I'm sure 1/2 syringe would be enough to correct the asymmetry.

Thank you for sharing your question and photograph.  I would recommend only treating your lips with a thicker hyaluronic acid filler like Juvederm Ultra Plus.  Often with lip augmentations the lower border of the upper lip will cover more of the gums and upper teeth of a patient to help improve their smile appearance.  Your smile asymmetry is so subtle I think this is all you will likely need.  Hope this helps.