how many systems are there?

[shinmeeok]

How many operating systems are there out there and which is the best overall?

[Bones92]

This many: http://en.wikipedia.org/wiki/List_of_operating_systems 

As regards to which is the best, it depends what you want.

[BC_Programmer]

"how many types of shirts are out there and which one is the best overall"

that's pretty much the same kind of question.

[Zylstra]

I tried witting an operating system once...

It was a horrifying interface... let me tell you that. Talk about "Anti-Multithread"

The most common operating systems follow in the lines of (Left to right, most popular to not as much, but still used):
Windows, Mac, Linux, Unix, BSD, BeOS.

[naiad]

Every one of those listed are Platforms with multiple individual OS's built on them.

[BC_Programmer]

Every one of those listed are Platforms with multiple individual OS's built on them.

a "platform" IS an OS.

when discussing linux, there are different distributions. but they are ALL called Linux, just as you can call any and all incarnations of windows "windows".