how much exercise does a whippet need in 1 day?

It all depends! A whippet will take almost any amount of exercise that you can give them other than loooooooooooong walks.
Basically a whippet is a dog that uses energy in short bursts. A long walk is NOT what is needed. A walk with a degree of free running is much more suitable for this breed.
Many show people are not of the opinion that free running is good for the dog particularly when young. We, however, disagree.Whippets were bred to walk and be released to catch game. That is what the exercise regime should be like.

I agree with Longdog,

they are so much happier when allowed free running. I give mine approx 1 hr a day off the lead in the park. They have access to the garden during the day.