When making a small box of pudding the amount of milk you use is 2 cups of milk per box of instant pudding.
You can add a bit more milk to thicken the pudding but adding 2 cups of milk to the pudding should be enough to make it thick enough.
If you don't add enough milk to the small box of pudding it will not be thick enough.
But when you add 2 cups of milk to the small box of pudding it should turn out just fine.
When making pudding from a box pour the 2 cups of milk into the bowl and then add the pudding mix and then mix it with hand or an electric mixer.
The pudding should set up pretty quick and you can also beat the pudding mixture with a wire whisk and then refrigerate it until ready to serve.