Any set of accounts defined in advance. Any number up to 110.
This simpler judging mechanism is nicer for getting a working implementation out sooner rather than later. But please make sure that the PM has trading fees defined (similar to the custom defined percent trading fees that UIA issuers will be able to set), and use that collected pool of BitAssets to reward the judges. This provides the incentive for the judges to bother submitting an outcome ballot for a PM.
The PM should not become active for trading until a quorum Q1 of the defined slate agree to be judges for that PM by signing a transaction stating that fact and providing a security deposit. Anytime before the PM becomes active they can take back their agreement and their security deposit, but after the quorum is reached, the security deposits become locked, the PM becomes open for trading, and the judges who submitted a security deposit become active judges for that PM. The remaining judges in the defined slate have 24 hours (or until the active judges reach a quorum that the outcome of the PM is known, whichever happens sooner) to add their security deposit (perhaps at a premium to make it fair to the existing judges since the new judges will have an information advantage: they might have gotten a glimpse at the trading volume of the PM over the first 24 hours which helps them estimate potential returns) to also be included as an active judge.
Later, when a quorum Q2 of the
active judges (the ones with security deposits) of the PM decide that the outcome of the PM is known (and make this known to the blockchain through a vote transaction), it would initiate a 24 hour period for the judges to submit a concealed commitment for their ballot. After the 24 hour period they would have another 24 hour period to reveal their ballot. If a quorum Q3 (<= Q2) of the active judges reaches a particular decision (yes or no), they get their security deposit back and also share the BitAssets collected in the pool for that PM as well as the security deposits of the judges who voted the other way and the judges who did not submit a ballot at all (either by not submitting the commitment in the first 24 hour period or the reveal in the second 24 hour period). I think Q1, Q2, and Q3 should be chosen so that they are all super majorities of some degree.
Also, the PM creator can optionally specify a date before which the active judges cannot claim the outcome of the PM is known even if they all unanimously agree that it is. This is to prevent the active judges from coordinating to all vote either yes/no to just get back their security deposit and the BitAsset rewards in the pool rather than having to wait (and incur opportunity cost) until the outcome of the PM is known.