The OnSync Enterprise Server License installation can be carried out on 1, 2 or 3 servers.
Here are some typical setups:
3 server setup:
Server 1: Apache, MySQL, PHP (Linux)
Server 2: Flash Media Server (Linux)
Server 3: OnSync Media Converter (Windows)
2 server setup:
Server 1: Apache, MySQL, PHP, Flash Media Server (Linux)
Server 2: OnSync Media Converter (Windows)
1 server setup: Apache, MySQL, PHP, Flash Media Server, OnSync Media Converter (Windows)
Note that the single server setup must run on Windows as the OnSync Media Converter requires a Windows OS. See http://www.digitalsamba.com/technical-specifications for more details.
Aside from performance under load, which is the primary argument for a 2 or 3 server setup (and the topic for another article) there is also the issue of making common ports available to multiple services.
FMS uses port 1935 by default to connect to clients. Many corporate firewalls do not open this port, but do keep standard ports, such as 80 or 443 open. By assigning ports 80 and 443 to the FMS service you can increase connection success for such participants. Since you can not bind multiple services to the same port on the same NIC, you will either require 2 NICs - one binding FMS to ports 80 and 443, the other binding Apache to ports 80 and 443 - or opt for a 2 (or 3) server setup.
In summary, if your group of users connect from behind many different corporate firewalls over which you have no control, you will reduce connectivity problems by ensuring that FMS can bind to port 80 and 443 as well.