This recipe that I've changed up from the Food Network says it serves 4 but honestly I think it's more like 2, strength wise.
6 ounces light rum
8 sprigs of mint, more for garnish if required
6 tablespoons fresh lime juice
4 tablespoons sugar
4 slices lime
Mix rum and sugar syrup in a jug and crush in the mint with your fingers.
Add your lime slices and pummel gently with a muddler (or do as I do and use a rolling pin).
Place ice into four high ball glasses and add ice.
Add 1/4 mix to each glass and fill up with soda water/club soda.
Garnish each with a slice of lime and a sprig of mint.