IIRC if you are not using Mach3 for profit/comercial use, then you can personally have as many copies in use with the same liscense file.
Art and now brian are very generous in the way it is liscensed.
They are in the process of changing how the liscensing is actually done, but they have said it will be just as generous. The change is to prevent people from buying one copy/liscense and then selling it all over the world.
Mike
Warning: DIY CNC may cause extreme hair loss due to you pulling your hair out.