For the POC phase:

Windows server, 4GB+ RAM, 50GB+ HDD

For Production:

Windows 2012 R2 or Higher Standard Edition/2008 R2 Ent. +, 16 GB RAM, Dual Core Intel Xeon E3 or similar, 5 GB free disk space (mainly for logs).

SSD with a R/W speed of at least 50MB/s.

One server supports up to ~7k concurrent users

Win2012 R2 or Higher, Quad Core Xeon or similar, 24GB ram, 25G disk space.

SSD/Raid with R/W speed of at least 100MB/s.

One server supports up to ~18k concurrent users

Win2012 R2 or Higher, 2.3 GHz Intel Xeon® 16 Cores, 64GB ram, 50G disk space.

SSD/Raid with R/W speed of at least 200MB/s.

One server supports up to ~40k concurrent users