I think there's supposed to be an infinite amount of stock available. That is, the futures market administration lets any participant buy a full combination of all possibilities at the full reward value, and then they can sell off the ones they think are overpriced so a market hoarder can't prevent such selling.

That being said, a manipulator may be hard to distinguish from someone with private information from the point of view of other participants, and I expect they would in fact influence the price.