If you have lots of video on demand , than your server needs to reach the content very fast and also you may need to use wowza media cache etc. in this position , you need SSD for fast response.Disk read and write speed is important for vod streaming.

For the live stream , how many different live stream will be broadcasted?

do you use transcode , transrate ?

if yes, how many live stream will be transrated . For the performance benchmark look at the wowza benchmark test.

This is classical maths problem.

your_client_count = your_server_upload_speed / (avarage_stream_bitrate)*(different_stream_count)

for instance in this picture

1 stream , no transcode , no transrate

bitrate : 200kbps

my server : up to 500Mbit upload with no traffic limit.

maximum_client_count = 500Mbit / 0.2Mbit * 1 = 2500 .

Like clustering your server , wowza media server and wowza streaming engine support live and edge technology, that helps us to use single stream from different server via the same connection ip. If you stream live hd around 1080p (2mbit upload per second ) without load balancing , you need the publish the same stream to different server , and management of the live stream and dist the users dynamically is very hard, it needs prepared connection address, however with the load balancer , dynamically your server connection counts equal.(50 over or above )