dinner party riddle

I guess the joke is how hard and simple this is.....

A professor and his wife have a dinner party. They invite 4 other couples, all of whom attend the party. During the party, everyone shakes hands with other people. At the end of the party, the professor asks everyone including his wife how many different people they shook hands with. Each person has a different answer (e.g. only one person shook 5 people's hands, etc.). Assuming that no one shook hands with their spouse, or themselves, how many people did the professor shake hands with?