Because every login requires a session, this parameter effectively determines the maximum number of concurrent users in the system. You should always set this parameter explicitly to a value equivalent to your estimate of the maximum number of concurrent users, plus the number of background processes, plus approximately 10% for recursive sessions.
12c (1.5 * PROCESSES) + 22
ALTER SYSTEM can be used in a PDB only to change the value of the SESSIONS parameter for that PDB. (仅限在PDB中修改)
For a CDB, the root container‘s SESSIONS parameter specifies the total number of sessions for the database.
the SESSIONS parameter does not count recursive sessions and hence does not require the 10% adjustment.
process --- 针对的操作系统进程(这里不做详述，专属和共享模式)，而session是db层，两个概念不同。
这里涉及到的试图有：(其中概念可以查看官档)
gv$session
gv$process
v$resource_limit
v$license
原文：http://blog.51cto.com/2012ivan/2107148