In a blender or using an immersion blender, combine 4 cups Thai basil leaves, ½ – ¾ cup lime juice, and 2 cups simple syrup, and blend until smooth.
Strain the mixture through a cheesecloth-lined colander.
Mix with sparkling water at a 1:2 (mix:water) ratio for drinks, and store any remaining mix in the refrigerator until ready to use.