Oracle recommend 3 (no more, no less) regardless of the number of nodes in the cluster.
I agree that 2 would seem logical for a 2 node RAC, but I actually use 3 no matter what the numbers are.
The SCAN does not point you to the node you will use. It is just a cluster alias that allows you to use a single address to contact the cluster. Any connections requests are handed off to the listeners in the normal way, so the SCA listeners are just an entry point into the cluster.