Hi Michael,
No of jobs depends on the no of background processes. You can use operation mode to limit background processes for day and night.
Referhttps://help.sap.com/saphelp_nw70/helpdata/en/c4/3a5e5c505211d189550000e829fbbd/content.htm
OR you can use a separate instance for background jobs with high no of background processes.
Also to restrict users on background job, you can use authorization objects. Referhttp://help.sap.com/saphelp_nw70ehp2/helpdata/en/86/f7f3393bef4604e10000000a11402f/content.htm
Hope this helps you..
Cheers
Divyanahu