It is actually advisiable to leave the setting at 5, which is the default.
You can however, add additional Job Servers through the CCM (Crystal Configuration Manager).
It is recommended that you have a 1 to 1 relationship between job servers and processors on the box.
So if you are running the job server on a dual processor box, you can add a second job server to the machine, each with a Maximum allowed Jobs of 5.
~Brian