There really is no fixed limit. It's more a matter of what you server can handle. You need to monitor server performance over several days. If you are only using peaking at 90% capacity, then you are probably at your limit. If you are only at 20%, then you can obviously increase the Job Limit.
If all jobs running on the agent place the same demands upon resources this is all you need to do. If there are some jobs that require significantly more resources, then you may want to use Tidal's Resources functionality to optimize utilization.
You can also use Queues to limit the number of high resource jobs running at any one time.
Thanks.