Let's understand how the Publication IDs are getting generated in the SDL Tridion: The publication IDs are NOT generated based on any algorithm or calculations but they are simply the
integer type data column with
Identity set to true - You can refer the
Publications table in the Tridion Content Manager Database. Now as the property of MS-SQL Server
Identity column suggests - If you create 10 publications for an example, you will have their Publication ID from 1 to 10; if you delete 4 last created publications - you will left with 6 publication with Pub ID from 1 to 6 and now if you create a 7th publication, the publication ID for this will be 11 - So creation & configuration of Publication ID is something in control of MS-SQL Server and not SDL Tridion.
Now how to control the
Identity columns is something should be raised to SQL Server community and also this is something none of us will suggest you to do considering it may end your SDL support.
However, if you are really interested in configuring/resetting the Publication IDs generated by
Reseeding the identity column, to help you with, you can (please note I am not recommending due to the reason suggested by Jonathan in the answer above) execute following command -
DBCC CHECKIDENT (dbo.PUBLICATIONS, reseed, 0)
Here the last parameter will specify the next Publication ID (in the above example: 0 means the next publication ID will be 1 provided there is no already existing publication with this ID)