/dev/hda1
 

When I use the df command, I am shown the following:
/dev/hda1 as boot and /dev/hda6 as /

Why is there no hda2, hda3 etc?

Why basically 1 for boot, then 6 for what I assume is the filesystem?

The missing volumes are not currently mounted or linux does not see them or recognize them and probably doesnt need to. You can print your partition table using
Now, take a look at your /etc/fstab and see what all you have entries for. Mount the ones present if necessary and check disk usage.

df only shows the moutned file systems. what they are was up to you when you installed the system. df tells you clearly where they are mounted anyway.

try running fdisk -l /dev/hda to see what all the partitions are officially listed as