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Are you asking what the maximum number of batch jobs is that any AS/400 can run? Or are you asking about the maximum capacity of your site's AS/400? Or are you asking how many are configured on your site's AS/400? Or are you asking about the authority you personally have to run batch jobs on your site's AS/400? Those four questions can have very different answers.
Large AS/400s can run many thousands of batch jobs at once with no problems. Your system can probably run at least hundreds at one time, but it would depend on the resources they have available. If the jobs don't interfere with each other, more can run without problems.
Your configuration might limit the number through a couple system values, through the number of batch subsystems you have, and through the job queues for those subsystems and their settings. Your personal authority might limit you to using particular job queues and that could limit your personal number of jobs.
Running multiple batch jobs at one time can take some thought. You don't want one job locking up resources that a second job needs. Many jobs actually need to be controlled so that they must run one after the other, rather than at the same time.
Subsystems can process jobs from multiple job queues. It's common to have one or more job queues that are "single-thread"; they allow only one job at a time. Different applications can have their own single-thread queues. There can be other "multi-thread" queues that allow multiple jobs at once. Jobs that don't need exclusive access can go through those queues.
It all depends on what your applications need and what your configuration is.