Does this mean that if the value of job_queue_processes is set to 5 I can only have 5 jobs scheduled for the same time.
This would be mean that you will only have 5 processes serving the job queue at any one time.

On my system, jobs are submitted by users for immediate execution (well, dictated by job_queue_interval) - to limit the number of processes that can run simultaneously I have set this to 3. Jobs that don't get there first have to wait for the others to finish.
I guess you can set it to what you think you require, but as you say, each will create a session and a process, so make sure your system can handle whatever you set it to.