The answer to this question depends on the answer to these questions:
1. What CPU are you using?
2. Do you have multiple processors?
3. In general how is your I/O performance?
4. What are the jobs doing?

If you have a multiple processors you can run multiple jobs with good
performance. A larger CPU
(9X9 or 99X etc) can handle more than a smaller (937 etc) one. If you are
I/O bound you will have more problems than if you are not.

The big question is what are these jobs doing? Running simultaneous jobs
that access the same databases or files will cause performance problems.
This is the main issue.

My suggestion is after you answer these questions, experiment with two then
three. There will come a point of diminishing returns. I hope this helps.