1) Depends where you buy the computer from. If you purchase an off the shelf machine, there is no one answer fits all, but typically you will have 10-20GB reserved for recovery files and may not have a drive letter assigned, and the rest as usable on the C drive, (If Windows 7, possibly 100MB for the system partition).
If building yourself, when you install Windows, you will be presented with a screen that allows you to partition the drive anyway you like:
2) There are many reasons for having more, usually people add more disks for more space. If you are talking about additional partitions (letter assignments) for the same drive, this is usually just done for organisation. (e.g. system files on one drive, personal data on another).
3) Yes you do, on Windows Vista or Windows 7 you can use the disk management tool to a basic level. It will allow you to shrink and grow partitions. For more advanced drive reassigning e.g. moving and growing, I would look at Gparted
4) Not at all, read 3. Recovery partitions are only used by some manufacturers.
5) When I first built a machine for myself, I thought it would be a good idea to have a partition for everything - one for system, one for work, one for downloads, one for applications etc. It is very hard to plan ahead and it is a waste of time! I then moved to a 20GB partition for system and the rest for data, however, I personally think it is a waste of time as many applications install to the C/system drive regardless and it is so much easier just to put everything on one drive!