The best number of partitions depends on how you use your computer and how you want to want to set up your hard drive.
I use three partitions:
1. Software - O/S and applications which I never back up as I have the original discs
2. Data - which I back up regularly
3. Video & Audio - which I also never backup as it is data that I copied from a CD or DVD so I have the original copy for backup
4. Backup drive
I prefer this method because I only have to reformat and restore the software on drive 1 if the system becomes corrupt and it simplifies and expedites the regular backup of info on drive 2. I use drive 4 to backup drive 2 data. This is in addition to regular offline backups. Backing up to drive 4 is easier and faster than doing an offline backup to flash memory or DVD and will provide a recent copy if a file is corrupt or inadvertently trashed, but off course is no help if drive becomes corrupt.
I used to have a fifth petition on which I loaded applications - essentially software other then the O/S. However, since some place data in system files, which means it would have to be restored if the system was reloaded, I combined the two.