I tried several counts (distinct) also for machine, client etc.. on V$SESSION (I think..)
Does anyone knows the underlying query of "number of clients" ??
in sql developer menu: view > dba > instance viewer
select count(*) as CLIENT_COUNT from ( select distinct process, machine from v$session where username is not null and type = 'USER' and machine is not null and process is not null )
If you use JDBC-Connections you can monitor with "View/Log" on Tab-Page "Statements" the SQL generated by SQL Developer