I'm trying to convert a small lathe to CNC and use it as a platform for 3D printing so it will need 4 stepper motors.
I'm new to stepper motors and CNC so I've been trying to learn. I got a lot of useful information and links from the Reprap wiki http://reprap.org/wiki/RepRap_Options
I am planning to use a piece of Arduino software called Sprinter (mentioned and linked on the Reprap pages) which needs an Arduino Mega as an Uno hasn't enough memory. From what I can see the Sprinter code is easy to follow. I've no idea whether a Mega would be able to do enough calculations per second to manage 8 steppers but I suspect it's taken some effort to get the program to work well with 4.
Of course if your machine only needs to run one or two motors at any one time The Arduino would probably have plenty of capacity.