Calculate number of concurrent users of an existing instance

The view v$license keeps track of concurrent sessions and users.

SQL> desc v$license

 Name                                      Null?    Type

 ----------------------------------------- -------- ----------------

 SESSIONS_MAX                                       NUMBER

 SESSIONS_WARNING                                   NUMBER

 SESSIONS_CURRENT                                   NUMBER

 SESSIONS_HIGHWATER                                 NUMBER

 USERS_MAX                                          NUMBER

 CPU_COUNT_CURRENT                                  NUMBER

 CPU_CORE_COUNT_CURRENT                             NUMBER

 CPU_SOCKET_COUNT_CURRENT                           NUMBER

 CPU_COUNT_HIGHWATER                                NUMBER

 CPU_CORE_COUNT_HIGHWATER                           NUMBER

 CPU_SOCKET_COUNT_HIGHWATER                         NUMBER

select sessions_current from v$license;

The above query will give you the number of concurrent users right now.

You can write a small job which will capture this information every hour for a week.  Once you have this data, you can take an average of this data to get the number of concurrent users.