I would build a PBN of 50 blogs or more if needed and then use it for multiple clients. If your clients don't know much about SEO then you just send the link report to the client. They won't out. Also, if you block link trackers such as Moz, Majestic, Ahrefs, etc ... then even their competitors won't find out about the PBN that easily.
Or if your clients are in luxury niches where they make a lot of money from search traffic then build 1 PBN for each client. Ultimately, it depends on your clientele and how big they want to dominate.