That’s a great question. Since it’s hard to make a direct correlation between number of users and the server load in a complex system like Discourse, it’s fair to acknowledge that the main bottleneck in a RaspberryPi system is the storage IOPS.
So as long as most of your needed resources are in the RAM, between proccesses RSS and Linux caching, you should have a smooth experience. The fact that Cloudflare is acting like a caching CDN will also help quite a bit, and you can even stretch the longevity of the Pi setup by using Using Object Storage for Uploads (S3 & Clones) after a while.