How to quota a CDB only tablespace to a CDB user?
All containers have a default tablespace:
SQL&gt; select p1.name, p2.property_name, p2.property_value
from v$containers p1 join cdb_properties p2 on (p1.con_id = p2.con_id)
where property_name = 'DEFAULT_PERMANENT_TABLESPACE' order by 1;

NAME       PROPERTY_NAME                  PROPERTY_VALUE
---------- ------------------------------ --------------------
CDB$ROOT   DEFAULT_PERMANENT_TABLESPACE   USERS
PDB1       DEFAULT_PERMANENT_TABLESPACE   USERS
PDB2       DEFAULT_PERMANENT_TABLESPACE   SALES

Create the user without specifying anything:
SQL&gt; create user c##myadmin identified by password;

User created.

SQL&gt; select p1.name, u.default_tablespace
from v$containers p1 join cdb_users u on (p1.con_id = u.con_id)
where username = 'C##MYADMIN' order by 1;

NAME       DEFAULT_TABLESPACE
---------- ------------------------------
CDB$ROOT   USERS
PDB1       USERS
PDB2       SALES

Then alter the user in the root container:
SQL&gt; show con_name

CON_NAME
------------------------------
CDB$ROOT
SQL&gt; alter user c##myadmin default tablespace admints quota unlimited on admints container=current;

User altered.

SQL&gt; select p1.name, u.default_tablespace
from v$containers p1 join cdb_users u on (p1.con_id = u.con_id)
where username = 'C##MYADMIN' order by 1;

NAME       DEFAULT_TABLESPACE
---------- ------------------------------
CDB$ROOT   ADMINTS
PDB1       USERS
PDB2       SALES

SQL&gt; select c.name, q.tablespace_name, q.max_bytes
from v$containers c join cdb_ts_quotas q on (c.con_id = q.con_id)
where q.username = 'C##MYADMIN';

NAME       TABLESPACE_NAME                 MAX_BYTES
---------- ------------------------------ ----------
CDB$ROOT   ADMINTS                                -1