What is the recommended value for Maximum Sessions?

As a best practice, the Maximum Sessions value needs to be set according to the maximum number of simultaneous active sessions that are expected to be opened.  To scope out the number of active sessions, run the following query in ScratchPad:

select * from  /services/databases/system/SYS_SESSIONS where status = 'ACTIVE'

Alternatively, the following query will give a breakdown by session owner, along the number of sessions for each owner:

select owner, COUNT(*) from  /services/databases/system/SYS_SESSIONS where status = 'ACTIVE' GROUP BY owner

Example output of the query:
OWNER         COUNT
system            2
admin             10
joe                  40
anna               20

In this example, the number of sessions is 72, so a Maximum Session of 100 would work. If the count were to be 1400,  a Maximum Session count of 1600 would be better (this would provide a fudge factor of 200 to cater to any unexpected spikes).