Is there a way to create NPCs within your created campaign? Since you only have so many character slots. Should the be created as Monsters? Any help is welcome. 

I prefer creating NPCs as monsters, or just straight up using an NPC stat block.

There currently isn't a way to add monsters to campaigns except in character extras.

I started to add them as characters but it only allows for 6, and I may end up with that many playing in the party, I was hoping to have another way to catalog all of those they encounter along their journeys. This would allow them to go back and read up on info on them throughout the campaign. Hopefully, it could be a consideration. 

If it is just a catalogue, then you can use campaign notes and have tooltips to official monster stat blocks you use.

Why not just create a PC, then have it unassigned. Granted you cannot have more than 6 without a subscription, but you could always make a new DND Beyond account in order to create PC's and just keep adding them

I actually had the same idea.  I've since updated to a Master subscription, but it still only allows for 12 characters per campaign, so it won't be a great use for their reference.  I'm hoping that this is on the development path or that there is a useful tool for this somewhere else in the community. 

edit: ( as I'd like to stay digital if possible).

Campaigns allow 12 users per campaign, with 99 characters per user

AH, so it is 12 USERS, not 12 Characters...  THAT is interesting.  Maybe I can start adding in NPCs...  But if my friends are just using free accounts, they will still be limited to their 6 character limit, right?

Quote from Matthew_Guillemette >>

AH, so it is 12 USERS, not 12 Characters...  THAT is interesting.  Maybe I can start adding in NPCs...  But if my friends are just using free accounts, they will still be limited to their 6 character limit, right?

 
Yes.