Like Vista they all come on the 1 DVD well at least all the 32 Bit Versions come on 1 DVD and all the 64 Bit Versions come on another DVD. Then depending on the Product Key that you use this either allows certain aspects of the OS to work or not. So if you where to use a Home Key you would get the Functionality of a Home Basic Version and if you where to use a Ultimate Key you would get the functionality of the Ultimate Version.
Here in AU M$ still is not selling Product Keys if you want to upgrade your existing version of 7 to something better you need to buy the full Product and use the Product Key that comes with that Product. In the US M$ Sells just the Key and a way to switch Product Keys and activate the additional bits that you have paid for. ;)
Col