How find out active database user connection in Oracle

Use either one:

SELECT   'Currently, '    ||
 (SELECT COUNT(*) FROM V$SESSION)   || ' out of '    || VP.VALUE    || ' connections are used.' AS USAGE_MESSAGE FROM    V$PARAMETER VP WHERE VP.NAME = 'sessions' ;

SELECT   'Currently, '    || (SELECT COUNT(*) FROM V$SESSION)   || ' out of '    ||
DECODE(VL.SESSIONS_MAX,0,'unlimited',VL.SESSIONS_MAX)    || ' connections are used.' AS USAGE_MESSAGE FROM    V$LICENSE VL;