The hardware configuration depends very much on the number of website members and visitors and also on website settings and target, so we cannot give you exact requirements. The most important thing is rtmp server connectivity.
We recommend servers with 100Mbps or 1000Mbps ports in well connected data centers (that have gigabyte connections with several providers).
A dedicated 100Mbps connection can host in theory a maximum of 200 streams at 512kbps each.
Number of users depends on communication logic.
If there are 2 way private video chat rooms where each user sees 1 other video and transmits 1 video, 2 streams per user will be required.
If there are video conference rooms where each user transmits 1 video and sees other 3 videos, 4 streams per user will be required.
To host hundreds or thousands of streams under 512kbps simultaneously, servers with 1000Mbps connection and 8GB of RAM are required. It's important to have the server in a reputable data center with many connections so all your users can connect to the server at good speed, from their different networks and providers.
We host all our streaming servers within the UK.