With instant coffee, caffeine measurements are based on the amount of the instant coffee powder used to make the drink. The caffeine content of coffee itself varies enormously.
According to the USDA*, 1 rounded teaspoon of instant coffee (1.8 grams in weight) contains 57 mg of caffeine. No matter how much water, milk, or creamer you add to the coffee - the amount of caffeine will remain the same.
The strength and flavor is greatly influenced by the amount of water mixed with the coffee powder.
Instant coffee is made by roasting and grind the coffee bean, and then extracting the flavor. This is done by placing the coffee in a solution of water and then dehydrating the solution (spray drying).
Sources: 2006 USDA Nutrition Database SR19 (link)