Yes, there is. And they will close the parks in stages.
Stage One: Day Guests (ie, people not staying at a Disney hotel) who have not already purchased tickets will be turned around at the Auto Plaza and told that the park has reached capacity. They can try going to one of the other three Disney parks.
Stage Two: Admission is limited to Resort Guests, Premium and Annual Passholders, crossovers and other Guests re-entering one of the Parks today. Guests using WDW Resort Transportation and Guests in taxis will be admitted.
Stage Three: The theme park is only open to Resort Guests using Disney transportation from their Resort or other theme parks or Annual and Premium Passholders. Shades of Green and Downtown Disney Resort Area Guests using their respective transportation will also be admitted.
Stage Four: Closed to all new Guests.
I assure you that if the park every reached enough guests that they even got to Stage 1 closing (which does happen around the Christmas season), you would NOT want to be there in the first place.