Oracle license for multiple servers

Do I need a separate Oracle license for each Virtuoso server, or can I use the same license on multiple servers?

That would depend on the type of OpenLink Oracle ODBC driver chosen ie Single(Lite) or Multi Tier.

The Single-Tier Licensing Model requires the acquisition and installation of a separate license for a given number of connections on each machine the ODBC driver is installed on.

The Multi-Tier Licensing Model is based on concurrent database connections, with license enforcement on the server side of the multi-tier architecture, installed on the database or a gateway server machine. With the generic ODBC client side of the multi-tier ODBC driver installed on as machine client machine (hosting Virtuoso) as required but only the licensed number of concurrent database connections are allowed.