Great! Once 9 ringwraiths have been slain in the campaign (very very very unlikely...) then you can't take any more ringwraiths.
To stop people taking multiple ringwraiths in a single force to prevent others taking them (booo!) then there is a limit of no more than 2 ringwraiths per force at any one time. For example, Paul may take Khamul in one force on his team and Rob could take both the betrayer and the knight of umbar in the other force for his team. At the same time Will could take the undying and a generic ringwraith (maybe on a fell beast
)
To stop people taking multiple ringwraiths in a single force to prevent others taking them (booo!) then there is a limit of no more than 2 ringwraiths per force at any one time. For example, Paul may take Khamul in one force on his team and Rob could take both the betrayer and the knight of umbar in the other force for his team. At the same time Will could take the undying and a generic ringwraith (maybe on a fell beast
)
40K - Dark Angels, Dark Eldar, Necrons; Lord of the Rings - lots; Bloodbowl - Dwarves, Wood Elves; SAGA - Anglo Danes & Welsh; Guildball - Morticians, Engineers; Dropzone/fleet - Shaltari