How to prepare sizing for Media Server Extended

2000 people doesn't mean anything. You'd have to provide information on what you're doing, if you're streaming video/audio, just video or just audio, none of that and just data etc. Do you expect a lot of in and out of the app by users? Does it talk to any external systems etc.

Mostly though you're going to need to think about bandwidth. If your math works out that 2000 connections for full services will exceed the capacity of the NIC, then upgrade the NIC. If not, then you'll likely be just fine. Network is the first to max out usually.