What settings control the maximum number of connections per user in Oracle 12c?
You might want to check the SESSIONS parameter, which controls the maximum number of sessions for the system: 

SELECT name, value 
FROM v$parameter
WHERE name = 'sessions';


https://docs.oracle.com/database/121/REFRN/GUID-52804B5A-164F-44F3-8980-F2593B58D807.htm#REFRN10197

You can also verify that there isn't a limit configured via user profile. Query the DBA_PROFILES view to check if a value is assigned for the SESSIONS_PER_USER parameter associated with your user:

select profile, resource_name, limit
from dba_profiles
where resource_name = 'SESSIONS_PER_USER';


Info regarding profiles:
https://docs.oracle.com/database/121/SQLRF/statements_6012.htm#SQLRF01310