In telling the twelve Apostles that he was going to prepare a place for them, Jesus was echoing what Moses did for the twelve tribes thirteen hundred years before.
Back then, before the tribes could enter the Promised Land, Moses sent surveyors across the Jordan to draw up twelve portions of equal value for farming and grazing. Then, Moes had the leaders of the tribes draw lots to fix the territories that would go to each of the tribes.
What Jesus promised the Apostles was only vaguely similar. As St. John wrote, "Eye has not seen, ear has not heard, it has not entered into the heats of men the things God has planned for those who love him.