查看oracle的最大并发数限制，可是查看v$license视图
v$license视图：
里面记录了Oracle最大的并发数以及当前用户的连接数，
官方文档有如下描述：
This view contains information about license limits.
Column Datatype Description
SESSIONS_MAX
NUMBER
Maximum number of concurrent user sessions
allowed for the instance
SESSIONS_WARNING
NUMBER
Warning limit
for concurrent user sessions for the
instance
SESSIONS_CURRENT
NUMBER
Current number of concurrent
user sessions
SESSIONS_HIGHWATER
NUMBER
Highest number of
concurrent user sessions since the instance
started
USERS_MAX
NUMBER
Maximum number of named users allowed
for the database
其实，该试图中的SESSIONS_CURRENT就等于select count(*) from
v$session where TYPE= 'USER'
show parameter license_max_sessions
//查看最大并发数，如果是0，则默认是无限制，但如果在初始化文件里说明了，就以初始化文件为主