You may be thinking of the family pack that they had with Win 7. There's no such thing (yet) for Windows 8. There are volume licensing options but you almost certainly didn't purchase those, or you'd know you did.
Well i have bought windows 8 Pro and i want to know how many pcs i can install it on, i remember it being 5 but i could have just made it up or dreamt it...
You're thinking of the Windows Store limit. You can have up to five devices (phone, tablet or desktop) running the same apps with a single Windows Live login.
But you can only install your copy of the OS on a single machine. The only exception to Microsoft's single-device clause in the EULA used to be with Office 2010 Professional, where you could use the same license for two devices that have the same username and are both registered in your name.