It is easy enough to use the magic loop method, once you have seen it it is logical.
There is a video explaining it and other methods on this page:
http://www.knittinghelp.com/videos/advanced-techniques
I saw that some circular needles are also sold in 40 cm length but that is still way to long to knit socks without the magic loop.
How many stitches depends on the yarn you use, which has to be knitted with a fitting size of needle, and maybe also the pattern you use.
Best search for wool and pick a pattern that uses the same size of needle in the gauge as the needles you have.
I prefer circular needles with the magic loop over double pointed needles, but both can be used for socks. You can even change from the one to the other if you want.
Added after reading Tantalise's answer:
You do not need long needles to knit with the magic loop method.
At the moment I knit one sock on a 60 cm needle and the other sock on an 80 cm needle. Both work well.
I also have a long needle, a 250 cm long needle, you can use it with the magic loop method, but the ones below one meter work better for the smaller projects.