Right now the limit is 8.
In theory the BeOS supports any number of processors, but as with most things you have to pick a limit somewhere and we picked 8. We could raise the limit by changing a constant in the kernel but it would serve no purpose.
Until we actually try to port to a machine with more than 8 processors (today there aren't any we're aware of at a price point we think is interesting), we don't see any need in raising the limit for the sake of marketing hype. And don't worry, when the BeOS runs on a 32-processor monster, you will hear about it.