How many Operating Systems allowed in a single computer [closed]

No extra RAM needed, just enough hard drive space (or hard drives) to hold them all.

There is no limit... using boot loaders and similar technologies, you can simplyfy the process - When I was a younger geek, I used to think it was cool having everything from Windows 3.1 to XP and a few flavors of Linux on the same machine....

...As I grew older, I realised it was just a waste of time and a maintenance nightmare.

If you want multiple operating systems, I highly advise you stick with one and explore technologies such as virtualisation which will allow you to run multiple Operating systems from within one without all the headaches.

As many as they can fit in your storage space. Your MBR can have up to 4 physical partitions, while each of them can have logical partitions in it. But you can actually boot some Linux distributions from an image files inside other partitions even with FAT or NTFS file formats, so as many as they fit. Note that not all operating systems supports the same boot methods, especially closed ones. RAM is only important for each individual OS, so it's depends of that specific OS minimum requirements.

I would imagine that as long as you have the hard drive space for it, you can partition and install as many operating systems as you like. The RAM requirements will vary depending on the particular OS, but the number of systems won't be affected. Using a VM will make this whole process a lot easier.

You can even find out how to install 145 operating systems if you want.

Infinite. Well at least by theory. You can have as many operating systems as your hard drive can handle. The smaller the size of the OS the more OS's you can have.

Older operating systems (such as MS-DOS) had certain requirements which could limit how many you could have on a single machine. I believe (correct me if I'm wrong) that the mentioned MS-DOS had to be installed on the first primary partition.

With modern operating systems there's no limit; you can have as many as you want provided you have the disk space to hold them all and a properly configured boot loader.

However, having worked with multiples OS's on a single machine I can say this is rarely worth the effort. A much better alternative is to have them running in a virtual environment; the requirements are slightly higher then (you need to have enough resources to run your regular OS and then enough resources to run the added OS within that).

Unless of course you're asking how many OS's can run at the same time on a single machine (all virtual)... RAM becomes the main limit then. But as CPU cores and disk space becomes shared, resources will get spread thinner and thinner and all VMs will be slower and slower. There's no theoretical limit on how many OS's you can have running as long as you have the RAM, but calculating it would require answering some questions like "what kind of OS". An old DOS requires far less resources to run than Windows Vista.