This is a subject that has been hashed over many times, so here is an attempt at a summary of what is discussed in the forum at: http://forums.zoneminder.com/viewforum.php?f=14
For 1-4 cameras, you can use a core2duo or better CPU.
10 cameras, you should have low end server CPUs (i.e. avoid i3,i5 etc)(Xeon is an intel server CPU. AMD has a variety of server CPUs that work well.
10-50 cameras you want a powerful server CPU, but still don't need to break the bank. Old server hardware can be got for <$1000, even $600 if you are lucky.
Above that you might just want to pay the developer's to get them to fine tune a system. But you can also give a try at it yourself if you wish.
Keep in mind that depending on how you configure ZM can make or break your systems. Trying to run 4 4K cameras with mocord on a core 2 duo will fail, but it might handle 4 1080p streams with record on the hi res stream and modect on the low res stream. These kinds of settings are variable. When in doubt browse the forums and wiki.
- http://forums.zoneminder.com/viewtopic.php?f=14&t=30973 100-150 Cameras