In the old days, that is before kernel 2.6.20, when Pata disk device names were supported a Pata can have 63 partitions and therefore able to accept 63 operating systems installations.
A Linux is seldom larger than 5Gb and its foot print is typically about 2 to 3.5 Gb after installation. So Linux wise a 200Gb is good for about 40 systems, assuming each partition is 5Gb large.
The kernel 2.6.20 and newer have abondoned the Pata device names and so all SCSI, Sata, Pata and USB hard disks can only have 15 partitions (16 devices name including the whole disk). A user can still use an older kernel to get the 63 partitions but newer Linux would not read it.
As hard disks are getting bigger there may be a need to get more 15 partitions out of it. The limit that I found, using standard technique, is 44 partitions as detailed here.
The 44-partition scheme is a viable one as I have been running it all the time and just filled up the disk with 44 systems recently. It does have the disadvantage that only 11 partition can be mounted after the hard disk is operational. Basically I have use each primary partition as an extended partition to hold 11 logical partition in each and unhide the primaries I do not wish to boot. One needs to nest a set of Grub menus to carry the booting scheme.
I have been using VMware which is inside the XP in my box. However I find a Linux installed inside a host (which is XP in this case) is like a prisoner in solitary confinement because it resides in a cells (typically 8Gb large) can only communicate with the host and not its fellow inmates (can mount partitions of fellow Linux).
VMware has its usage but for studying a Linux, its components and its properties the virtual machine management layer is hindrance. In my opinion there is no way I can understand how Linux works unless I install it normally in a partition of its own, so that I can mount it partition, chroot into it, salvage its files, rescue it boot loaders etc, etc.
For those interested in having "a few" operating systems in the PC the following information may be relevant.
(1) All NT versions of Windows, Lilo and Grub can do multi-boot. Windows way is 10 times and Lilo way is 3 times harder than Grub.
(2) All MS systems if installed alone, all Solaris and all BSD systems (with the exception of NetBSD) need to be installed in and booted from a "primary" partition.
(3) All Linux can be installed and booted from any partition. That means put Linux and its swap in logical partitions and save the primaries for those that need them.
(4) Each hard disk can have a maximum of 4 primaries. If a logical partition is used one primary must be given up to become an exteneded partition. All PC systems have been written to accept only be one extended partition in a hard disk.
(5) The easiest way to multi-boot is to have a boot loader installed inside the partition where the OS resides. One can then use the boot loader controlling the MBR to boot each system by booting up its boot loader. Thereafter it is up to the second boot loader to fire up its master. This is a technique known as chainloading used by every boot loader. Every Linux, be it using Grub or Lilo, boots a MS Windows "always" by chainloading because neither Grub or Lilo can read a ntfs partition.