Hi and Welcome (I'm pretty new here myself),

You can run 5 machines (Some prefer this), you can run 1 machine and 5 accounts (Need a decent system, some prefer this as well).

If you go all hardware you can get a KVM switch which if I understand it right, allows you to use 1 keyboard/mouse for multiple machines, that said most hardware boxers from what I gather also have back up keyboards/mice per machine (Though I haven't figured out why yet).

This set up you wound need 5 (Or more) PC's and 5 Wow accounts. (You can get further info in the hardware tools forum here)

If you choose to MB with software, I'd reccomend Keyclone over the others (it doesn;t get any more user friendly, and the customer service that KC provides is well worth the $20 investment). There are free versions, but those weren't for me and for the least headache Keyclone was the way. Check Software tools forum on this site for further info.

With this route you can use 1 or more machines, 1 or more monitors.

My setup (Not as fancy as some here lol) but I am running 2 pc's/3 monitors and 5 wow accounts and Keyclone (1 per pc) and I have found this fits my budget for the moment.

I will tell you that there is a ton of info here, some needs decyphering (for me anyway the macros give me fits), but it is worth it when you cut through mobs like butter. I will add that there are times (I did) you hit a plateau and it gets boring, but then you move up 10 levels and can go new places so it gets fun again.

The New Multiboxer/support forum here is a great resource and this link Beginners Guide 2 Dual-Boxing (3,4,5 boxing) is a great start to help you out.

Like I said I'm fairly new here (Though I do read almost all the posts), but if I can help out I'm glad to do so.