well the first time you do a gauntlet you have to stop after completing it. Just programed in the game that way. Then, to get all 4 stars you have to play again and depending on the condition to get said stars can take multiple runs. Sometimes its get a win streak of 12 which means you have to play the entire gauntlet 3 times. Another might be kill a certain amount of a specific enemy (usually cursed) and the game only gives you about 1/4 of the needed amount per full run. dont forget the loading times, i believe each mission takes minimum of 5 minutes to complete with loading.
*im almost done the gauntlet and i find there are 2 more after those 3 for a total of 15 rifts. I am missing one at the moment. lv75 rift.