NIC is Network Interface Card. If you have a 1gbs nic, you have 1gbs of throughput. Actually you have about 80-90% of that, you can estimate 800mbs per 1gbs nic. Some nic cards have 10gbs throughput, but Wowza is a Java application and there is a Java limit of 5gbs.
It is difficult to estimate capacity for a one-to-few chat application because each additional user in a group increases the load exponentially. I.e., the load added by the 4th chatter is greater than was added by the 3rd chatter, because they all publish one stream and playback all the others' streams. The size of each group is limited by client capacity, most users can only publish and display a few streams.
If you are not managing the hardware, and are just using a service, the bandwidth number they give you is what counts. You will not be dealing with a NIC, which is hardware. The bandwidth they provide is what they are saying is your capacity or throughput with their service. The term "throughput" is synonymous with bandwidth: How many bits per second.