As of right now, just one, but progress on more is ongoing. But you're missing the point, it's not worth it to target the entire RPi supply chain at this point, and even if you wanted to target individuals, less than 0.01% of RPi products likely will end up as SeedSigners, so who knows which ones to pick. Whereas, at a certain point in a HWW's life, it's known that the device will be used to store PKs, and an attacker further knows that just about every device is going to someone who will potentially use it for that purpose. Different tradeoffs.