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. (僅限在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 --- 針對的操作系統進程(這裏不做詳述，專屬和共享模式)，而session是db層，兩個概念不同。

這裏涉及到的試圖有：(其中概念可以查看官檔)

gv$session
gv$process
v$resource_limit
v$license

Oracle process和sessions 解釋