It’s going to really depend on how popular it turns out to be. 
I would say you might need at least an 8GB/4vCPU with 50GB disk (depending if you begin with blank slate or you will migrate data into it from day one in which case you might need more) and 2GB swap to kick off, but make sure you have the ability to scale this if needed, especially disk space of the primary disk.
That should be able to serve up to 8 requests (not sessions) simultaneously if you configure 2 Unicorns per vCPU (ie 8). Other requests will be queued (and that’s ok if its just one queued rarely and two is very rare).
Discourse utilises a front end framework, and once that’s loaded, 99.9% of the data travelling between client and server is json, not full render, so it’s fairly economical with bandwidth and thus turnaround of requests.
You can then monitor and scale as required. Backing up and restoration is a doddle thanks to Discourse’s helpful UI and command line tools.
I’m sure some more specialist SA’s can chime in but that will probably get you very far to begin with.