I assume you are asking about a multi boot system. To have even more flexibility in the number of OSs, you should consider having one host operating system that will manage virtual, guest operating systems. This will allow you to run these OSs at the same time, rather than one at a time on the same host computer.
Generally, I agree 100% with JorgeM; however, to answer your question, there are a couple of factors to consider:
OS support for primary vs. extended partitions
Changes to OS where the boot loader is hosted
Confused yet? :-)
Windows can (or used to) only support installation of the root OS on a primary partition. Linux is not so encumbered - there are only 4 primary partitions in a normal MBR (Master Boot Record - the first sector on the system disc). One of those can host extended partitions. Windows (or used to) only boot from a primary partition on the primary system disc. Linux is not so encumbered (it can boot from any partition on any attached drive, including USB thumb drive).
The MBR has some low-level code that looks for the actual boot loader, which if you have a mix of Windows and Linux systems will usually be one of the Linux systems installed. That will have some data that configures the boot menu, and boot options for the various operating systems you have available, such as where they are located, etc. However, if you remove this operating system that hosts the boot loader, you will have to re-install the boot loader in another OS. This is not always simple. Caveat User!
So, as you might see now, doing multi-boot with even as few as 2 operating systems can be really complex, which is why it is preferable to use a virtual machine manager such as VirtualBox, and run other operating systems in a VM. This is what I do. I only have one primary host operating system, and if I want to run another OS, I run it in a VM. Much less painful, I can assure you!
Re. Caperjack. There may be a boot manager that could allow that, as long as the OS's in question can boot from an image file or an extended partition, otherwise you are limited to 4 (max primary partitions on standard PC MBR - not such a limit with newer boot records).
Starting, I belive, with XP, only Windows' boot files must be on a primary partition ["system" drive must be primary], but the remainder [the "boot" drive], of the OS may be on a logical volume. W7 has the option of a System Reserved primary for all boot files and loader.
Look, I didn't come up with the system and boot drive labels. M$ did.