Suppose we have a server that has only 1 CPU with 1 core, so we need 1 Oracle database license for installing a database into it. Now, if we would need to add a second server, combine them into cluster and install database into it, then how many licenses do we need - 2 database licenses and 1 Clusterware licenses or 1 database license and 1 cluster license? I'm not sure, how Oracle treats joined into Oracle cluster servers - as one server or still as separate servers? Because we would need to buy expensive Oracle cluster software, then it would be logical for Oracle to treat joined servers as one.
Oracle database licenses are quoted per processor, not per machine. They don't care whether you have two machines with two cores each or four machines with one core each. Both equate to 4 licenses you need to purchase.
For clustered installations, you need to purchase the appropriate database licenses and the RAC licenses.