There isn’t a fixed “safe” number of outbound links per PBN site. What matters more is how natural it looks. On most PBN posts, people usually keep it around 4–10 outbound links, with only one link going to the money site and the rest pointing to authority or neutral websites.
The key thing is avoiding patterns. If every post has the same number of links and always pushes to similar sites, it becomes easy to spot. It’s better to mix things up, sometimes having fewer links, sometimes more, and even a few posts with no outbound links at all.
Don’t focus too much on a strict number. Focus on making the site look like a real blog, with natural linking and good content, and you’ll be much safer.