Job number is stored in the F986111 table. You can reset the job number here. This is a Server map table so make sure you access the correct schema , depending on which server's job number you want to reset. There is no front end app to do this so you have to use an SQL update, generic example below
UPDATE <server_map_schema>.F986111 SET JCJOBNBR = 1 WHERE JCEXEHOST = <server name>
Before doing the update also check that resetting the number will not cause duplicates , depending on how much history you are retaining.