SQL> SELECT SYS_CONTEXT('USERENV', 'SESSIONID')
  2    FROM dual
  3  /

SYS_CONTEXT('USERENV','SESSIONID')
-----------------------------------------------
28738608

SQL> declare
  2   sid_ number;
  3   serial_ number;
  4  begin
  5   select sid, serial# into sid_, serial_
  6   from v$session where sid = (select sid from v$mystat where rownum = 1);
  7   dbms_output.put_line('Session id is ' || sid_);
  8   dbms_output.put_line('Serial# is ' || serial_);
  9  end;
 10  /
Session id is 139
Serial# is 25

PL/SQL procedure successfully completed.

SQL> declare
  2   sid_ number;
  3   serial_ number;
  4  begin
  5   select sid, serial# into sid_, serial_
  6   from v$session where audsid = userenv('sessionid');
  7   dbms_output.put_line('Session id is ' || sid_);
  8   dbms_output.put_line('Serial# is ' || serial_);
  9  end;
 10  /
Session id is 139
Serial# is 25

PL/SQL procedure successfully completed.