For a champion there is no limit.
The Contestant who was a winner comes back until they lose. In a tournament a losing contestant can sometimes continue to play by earning a wild card spot.
A Champion is invited back to pay in Tournaments and chosen. They can lose tournaments and still come back and play in anew tournament or even be chosen for a wild card spot and continue to play in a tournament after they lost.
A champion like Ken Jennings can even have tournaments that are merely special matches created just so they are a contestant, like the Watson Match.
So Contestants can play until they lose, Champions come back based on a set of rules created by Jeopardy for an event or can just be chosen for an event.
three times. ECW CHAMPION,WORLD HEAVY WHIEGT CHAMPION,UNITED STATES CHAMPION
28 times
Chris Benoit has been champion about 5 times
John Cena has been the World Heavyweight Champion twice, the United States Champion three times, the WWE Champion 11 times, the WWE Tag Team Champion two times, and the World Tag Team Champion 2 times. Altogether he has been champion a combined 17 times.
12 times, 10 time WWE champion, 2 time World Heavyweight Champion
he was world champion from 1919-1927
6 times
4 times
3 times but he is going to be world champion again this year.
Hulk Hogan has never been the World Heavyweight Champion, but has been the WWE Champion 5 times.
9 times
35 times