1. From 20c on, the "Non-CDB-architecture" will not be supported anymore, single-tenant i.e. CDB-architecture with one PDB will obviously be supported.
2. From 19c on you can have up to 3 PDBs on all editions of the database (SE2, EE), Quote from https://docs.oracle.com/en/database/oracle/oracle-database/19/dblic/Licensing-Information.html#GUID-0F9EB85D-4610-4EDF-8… : "For all offerings, if you are not licensed for Oracle Multitenant, then you may have up to 3 PDBs in a given container database at any time.". This license change was not "backported" to Oracle 18c.