Search engines give more value to websites that constantly have new content.
I believe that having at least one article a month would be good enough to keep the websites fresh but remember, it is important that the content be high quality. Don't throw bunch of articles at once.
As the content tends to be costly factor of a PBN, check whether the domain is clean and has no penalty before spending time and money in content.
After checking with the penalty, you have to use the links judiciously. There should only be one article pointing to a money site. You should not link out to your money sites in all the articles.
Each of your articles should also have 2-3 other outbound links to authoritative, non-competitive web pages. So Google takes note that your money site is linked alongside authoritative web pages.
Feel free to ask me more if you're not satisfied with this.
Best Wishes - BB