With a 1g nic (20% overhead accounted for.) = 800mbs throughput, which is 800 1 mbs streams, or 1600 500kbs streams.
You can use this formula for calculating:
number of users * stream bitrate + 20% IP overhead = total server bandwidth
You will have to test your actual server/network using the load test tool, or monitoring utilization in production. You must be properly tuned. And to achieve maximum levels you may have to tune your server and network in other ways outside Wowza.