I have seen a confusion between sequence and system change number quite often.
The sequence is a fairly little number which is the number of log switch since database creation (it can be resetted by open resetlogs).
The thread is only relevant in RAC. In single-instance database, it is always 1.
You can find the current sequence number with the following query
select sequence#,thread# from v$log where status='CURRENT';
The system change number (SCN) is a much higher number, which is continously increasing, even when you do nothing. The dbms_flashback package has a function to return the current system change number :
select dbms_flashback.get_system_change_number from dual;
In 10g, there is a standard function to get the current system change number
select timestamp_to_scn(current_timestamp) from dual;
Before a major application/database upgrade, it is good practice to make a backup and write down the SCN for an easier recovery procedure.
read my comment about non-accuracy of timestamp_to_scn