SCN is known as System Change Number, with the help of SCN you will be able to order database events, and you can check in what way the advance of the given logical stamp is controlled. Oracle database uses a system change number (SCN) which is a logical, internal timestamp. SCNs will command the events that occur inside the database, this will be useful for the satisfaction of the ACID properties of a transaction.
If you want database to query and track changes you will have to use SCNs. Let us have an example, if a transaction sometimes updates a row, at that time the database records the SCN at which the update happened, remember the other changes in the same transaction will have the same SCN, As soon as the transaction commits, the given database will record an SCN for the same commit. Numerous transactions that commit at the same time would be able to share the same SCN.
SCNs occur in a monotonically growing order, and there can be a huge amount of upper limit that will show that how many SCNs an oracle database would be able to use. You will always get an upper limit, please remember all the oracle database would not be able to run out of a given SCNs. If you want to check whether the same thing happened or not the given oracle database use a rationing system that depends upon the time.