This is for BIOS based computers:

http://en.wikipedia.org/wiki/Disk_partitioning
Extended partition

A hard disk may contain only one extended partition; the extended partition
can be subdivided into multiple *logical partitions*. In DOS/Windows
systems, each logical partition may then be assigned an additional drive
letter.

www.pcguide.com/.../structPartitions-c.html
One of the four partitions may be designated as an *extended DOS partition*.
This partition may then be subdivided into multiple *logical partitions*.
This is the way that two or more logical DOS volumes can be placed on a
single hard disk.

UEFI based computers can have up to 128 primary partitions.

This is for my IDE and SATA drives. I am not sure about SCSI or HFS, etc.
You can have any number of logical partitions. I have seven or eight at a
time. But it gets complicated once you get to 23 logical partitions in
terms of naming them. It is beyond my skill.

If there is an easy way to squeeze more in then let me know. :)