There are 5.047 .750 Litre bottles in a gallon
It would all depend on the size of the individual bottles.
A typical serving size of wine is 6 oz and there are 128 oz in a gallon so you can get about 21 or 22 glasses.
15 six oz glasses
With 15 gal of wine you will be able to fill 75 bottles (750ml) Cheers:) VinoEnology.com
From a regular 750mL bottle of wine. You are looking at 5 1/2 servings per bottle