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$sessiongv$processv$resource_limitv$license