Can you have more than 65536 sessions on an Oracle RAC database?
Each Oracle instance in a RAC configuration maintains its own resources and sessions; you would be able to have up to 65536 sessions for each node in the cluster.
https://docs.oracle.com/database/121/RACAD/GUID-59DEC066-5743-4EDE-9A02-E8896005F658.htm#RACAD818