Reset the numbering at the tabs of the Microsoft SQL Server Management Studio
The numbers represent the process id of the session associated with that tab - as every time you open a new tab it opens a new connection to SQL Server for you. They are not necessarily sequential but they can be. If you run the following command you should be able to see an entry for each of the tabs/connections that you have open:

select * from sys.dm_exec_sessions


I hope this helps you.