Different people take different approaches:
1. Create PBN sites that are multi-niche where content from sports or finance through to shopping and business could be published. Generally a persona is created and a blog style of writing and posting is used to fit all of the different topics in.
2. Create PBN sites that are focused tightly around the niche of one of your money sites.
You also need to consider that having a PBN of, let's say, 30 sites and linking from those 30 sites to the same set of, say, 5 money sites is going to leave an obvious footprint. Personally I only use each PBN site to link to one money site and this is working well for me.
But then I get all my domains for registration fee so it's not too costly. If you are buying expensive auction domains then you might have to do a mix of the two options above - in other words create a fairly generic site about a top level niche such as "health" which would allow you to link to diet, sports, exercise, etc while still having some benefits of being a niche focused site.
