What does "single JCL" mean? If you are referring to a sequential file or a member of a PDS, you may have any number of jobs in that file. When they are submitted, each job is treated independently by JES and assigned its own job number and run separately. If you are referring to something else, you will have to explain what you mean.
As far as JES is concerned, a job that is being input starts when a JOB statement is found and continues until any of several conditions are met.