Five gallons is 18.9271 liters. You will need to top up with
1.0729 liters of wine or 1 quart (32 ounces) plus 4 ounces. If you use 750 ml bottles, that will be
1 1/2 bottles of wine to top up this barrel.
Six gallons is 23 liters. Wine in small barrels doesn't take long.
Check for taste in 1-2 weeks!