The constitution doesn't set a limit on the number of members who sit on the Council and this is left to the discretion of the Monarch.
The King in Privy Council has the authority to make appointments to most posts in the judicial branch of government.
[1][2] One of the primary goals of the constitutional reforms of 2010 was to ensure the separation of the executive, legislative and judicial branches of government.
They chose to vest the power to make these appointment in the office of the constitutional and non-partisan head of state, who is the Tongan monarch.
The King in Privy Council is the final court of appeal for cases dealing with hereditary estates and titles.