It depends entirely on the platform and infrastructure. OpenACS supports clusters of webserver front-ends to the database (to some degree at least), so you could have a half-dozen dual-processor x86 Linux boxes with a couple GB each hooked up to a massive Solaris Oracle server and handle a huge load.
It is more realistic to ask:
1. How many users can I expect to support on this specific platform/infrastructure
2. What do I need to support N users or hits-per-second or ...
