If you are building links through PBN's then how many links would prefer from them, either private or public?
At the present day conditions. It is most evident that you don't even bother to get a link from public networks. And coming to the private ones, one link per site is an optimum number, no more or no less (obviously).
You just stick to the one site one link policy. Its the safest.
don't use PBN nowadays for link building, google is smart enough to know the link chain from the money site.