Franck Pachot is pointing this out in a community discussion:
You don't need an option when you have only one pluggable database.
You can detect violation with:
select name, aux_count from dba_feature_usage_statistics where name like '%Pluggable%' or name like '%Multitenant%';
If AUX_COUNT is >1 then you need to buy the option, or drop the additional PDBs
In 12.2 you can set MAX_PDBS to 1 to be sure nobody creates more than allowed.
Here are the features you can use in this configuration (know as 'single-tenant'): http://www.slideshare.net/pachot/12cr2-singletenant-multitenant-features-for-all-editions
In Oracle 19c, the MAX_PDBS can be set to 3 without breaking the license.
