Since you are asking in the black hat section, my answer is that you add as little content as you can whilst still avoiding a red flag with Google - so no single page sites, but you can probably get away with between 5 and 10 unique articles per site - no need to update once you reach this sort of level.
If you were to ask in the white hat section, then I might have said that you should try and build each PBN site into a mini authority in its own right, but I would only do this if you are in a competitive niche or you are going all out by building a single, large money site and you want to reduce the risk of it going down the pan in the long term. These types of PBN sites will require more content at more regular intervals.
But then, I am attempting to stay ahead of the game, so I am beginning to use the latter strategy for the majority of my PBNs now.