The amount of processes that are typically on a computer depend on the operating system and applications that you are using; on average you will have 40 to 50 processes.
No simple answer exists for how many processes should be running. There are a range of background services, that can be active at any time, which start and stop.
if you start the system, at any time you press alt + ctrl + del at same time, a task manager is opened, just go to process tab, its shows how many process should be running on your computer.
"Should" is a tricky question. I recently customized a build of Windows XP so that it can run with 5-7 processes, but that's with network processes turned off. It depends what you want to do with it.
