how many linux os do I need?

I would agree that you only need one, but if your machines are a mix of some old and some new, you might want to look into distros for older machines (which I am trying to do) simply because they're usually stripped down a bit and the hardware requirements are less. You want to obviously have the machine running at optimum.

As for as ease of loading, Mint or Ubuntu is usually problem free. If you're new, I'd stick with those. And Ubuntu has a variety of distros, one being Lubuntu which might run on older machines better since its their light version. Haven't tested it, yet.

I am, in fact, looking for distros for older laptops. You can install probably any distro on the laptop (I have old Thinkpads, and they're very good with Linux), however, the speed of the machine would probably be doggy with some of the newer, hyped-up distros.

And lastly, even with my older Thinkpads, which have a record of being Linux friendly, I've had problems installing with some of the Not-so-well-known distros, like Linux Lite, sadly. It looked great in LIVE DVD, but the install, which should have gone flawlessly, refused to boot up after I installed it, and their 32 bit wouldn't even install at all. Died in the beginning.

That's why, unless you have a lot of experience with installations and Linux, I'd stick with the more popular, stable distros.

(PS: I love Linux, and have been using Unix or Linux since they were born. Not trying to diss the operating system, just wanted to be truthful that not everything is roses all the time with installations...)