OK had a look at the sourcebooks and the 9 ringwraiths are named as:
The Witch King; the dwimmerlaik; the tainted; the betrayer; the knight of umbar; khamul the easterling; the undying; the shadow lord; the dark marshal.
So updating what I said above:
For the sake of simplicity, the you may pick an unamed ringwraith if listed in your army list and they count as a generic hero. However, as per the campaign rules, once it survives a battle and rolls on the chart it's treated as a named hero and you'll need to name it, but you can't name it after one of the already named ringwraiths in the game (eg the tainted) - give it another name. This represents one of the ringwraiths just being known as something else, they are very shadowy figures and could easily be given different names in different places just like the maiar known in some places as gandalf.
Besides that, if it becomes a named hero, or any of the named ringwraiths are slain in game and then roll a 1, they are for all intents and purposes slain and you can't use them in the campaign again. In 'reality' they are not destroyed but their regeneration time is longer than the campaign lasts.
Regarding keeping track of all the ringwraiths, you may take any of the following as allowed by your army list: The Witch King; the dwimmerlaik; the tainted; the betrayer; the knight of umbar; khamul the easterling; the undying; the shadow lord; the dark marshal.
However once selected by someone, no one else can use that hero. As Angmar is not in the campaign this only concerns the betrayer, the knight of umbar (both of which can be taken by Harad or Mordor) or Khamul th eeasterling (can be taken by Easterlings or Mordor). I think it's fair to say Mordor can't pick these three for the campaign and they are reserved for Harad and the Easterlings respectively as Modor has all the rest to choose from. You OK with that Will?
The Witch King; the dwimmerlaik; the tainted; the betrayer; the knight of umbar; khamul the easterling; the undying; the shadow lord; the dark marshal.
So updating what I said above:
For the sake of simplicity, the you may pick an unamed ringwraith if listed in your army list and they count as a generic hero. However, as per the campaign rules, once it survives a battle and rolls on the chart it's treated as a named hero and you'll need to name it, but you can't name it after one of the already named ringwraiths in the game (eg the tainted) - give it another name. This represents one of the ringwraiths just being known as something else, they are very shadowy figures and could easily be given different names in different places just like the maiar known in some places as gandalf.
Besides that, if it becomes a named hero, or any of the named ringwraiths are slain in game and then roll a 1, they are for all intents and purposes slain and you can't use them in the campaign again. In 'reality' they are not destroyed but their regeneration time is longer than the campaign lasts.
Regarding keeping track of all the ringwraiths, you may take any of the following as allowed by your army list: The Witch King; the dwimmerlaik; the tainted; the betrayer; the knight of umbar; khamul the easterling; the undying; the shadow lord; the dark marshal.
However once selected by someone, no one else can use that hero. As Angmar is not in the campaign this only concerns the betrayer, the knight of umbar (both of which can be taken by Harad or Mordor) or Khamul th eeasterling (can be taken by Easterlings or Mordor). I think it's fair to say Mordor can't pick these three for the campaign and they are reserved for Harad and the Easterlings respectively as Modor has all the rest to choose from. You OK with that Will?
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