From what I learn this far, inodes are the maximum file (and directory) you can have in single partition. You can fill the whole disk inodes without actually filling the disk space, or you can fill the disk space with one very big files, leaving inodes unused.
This question has come into my mind recently: where are those numbers coming from?
You did not mention a specific file system so I am going to assume ext4, although what I am saying should mostly apply to ext3 as well.
The number of inodes is determined when the file-system is created. File-systems are generally written to be flexible enough so that this number can be specified at creation to better suit the needs of the system. So if you have a lot of small files you can create more inodes and if you have a smaller number of large files you can create less inodes.
With mkfs.ext4 you can use the -i flag to specify the bytes per inode. The default value as of now is typically 16384 bytes per inode. This number is nothing specifically special but if you assume the typical 256 bytes for the inode size and 16384 bytes per inode you get approximately 1.56% of the disk space being used by inodes.