In the case of the server, specifically KepserverEX, you can have multiple channels running multiple connections. Think of the server as the means to connect the device (PLC) to the client (Software) If you have multiple devices the server acts as the means for connecting these devices. This is a software product. You will still need to address the phyical (hardware) connection from the device to the computer. The server enables the different protocols to speak to the client.
For instance if you are using the Allen Bradley Connectivity Suite, you can utilize all of the protocols available in that suite, whether Ethernet or Serial. Many of the Connectivity suites offer Ethernet connections so long as they are available in the manufacturer's hardware. A listing of the suites and available connections is available through the following link:
http://www.eternity-sales.com/Kepware/kepwaredownloads.htm
If you have devices from different manufacturers, you will require connectivity suites for those manufacturers as well.
In terms of what you are looking for will depend upon what your requirements are and therefore being able to address those requirements.
You will want to first determine if your client (HMI Software / Database) is OPC capable.
Second you will want to obtain a list of devices your customer is looking to connect to the client.
From there, we can determine the items (connectivity suites) you will require for this application.
So, more information is needed before addressing what is required.