To add to Sasha's post, it also depends on what kind of machine you are running.
A fast machine may be able to handle a few more of these programs running in the background, but even the fastest systems can be slowed down considerably by using too many of these programs. So you really need to find the right balance beetwen performance and security for your machine.
I have a pretty decent Core Duo system(pretty well OCed aswell) with 2gb's of ram and even this quite powerful machine get's slowed down considerably if i put on too many of these programs. ATM(i am still deciding on my security setup) i am running 3 resident antimalware programs, an AV and a firewall and the machine handles it with no problems but i am pretty certain if i would put all these programs on my other machine(stock barton 2600+ with 1gb ram) it would slow it down to a crawl.
BTW don't worry about your english, it's fine ...