To understand the difference between 1 Simultaneous Driver Usage and Unlimited Simultaneous Driver Usage, please read Supported Device & Driver Plug-in List for NI OPC Servers. Every element within that list corresponds to one of the drivers supported by NI-OPC Servers.
If the 1 Simultaneous Driver license is purchased, only one of these drivers can be used at a given time. It is possible to change between drivers when needed, though. You will still be able to communicate with several devices simultaneously, as long as they all are supported by the same driver.
With the Unlimited Simultaneous Driver version, several drivers can be used at the same time. With this configuration, you will be able to communicate with devices supported by different drivers simultaneously.
NI provides the OPC Servers as a separate purchase, for a simultaneous driver usage of a single driver or for unlimited drivers.