It will vary on the size of your teacup and your preference. As per Telegraph.co.uk, for a 200ml of freshly brewed tea you need to use 10ml of milk to achieve that perfect cup. Therefore a liter of milk can make around 100 cups.
Instructions for perfect cup of tea for one:
1. Add 200ml of freshly boiled water to your tea bag (in a mug).
2. Allow the tea bag to brew for 2 minutes.
3. Remove the tea bag.
4. Add 10ml of milk.
5. Wait 6 minutes before consumption for the cuppa to reach its optimum temperature of 60 degrees centigrade.
Here's a list of the essential "dos and don'ts" of Afternoon Tea shared by Afternoontea.co.uk.