Needles in a set of double-pointed needles

Dear users, dear authors,
As far as I know, a set of double-pointed needles contains 2–5 knitting needles? What is the purpose of each number of knitting needles?

Hello Josefa and dear readers,
a “classic” set of double-pointed needles consists of 5 needles. The needles have a point on both ends. They come in metal, wood, bamboo, carbon fiber, plastic… in many different materials, and choosing the best material for a specific project is a science in itself. Most of the time, however, it’s purely a matter of personal preference which material you prefer to work with. I prefer wood.

As far as I know, there are two lengths: 15 and 20 cm, and they come in sizes ranging from about 1.5 to 7 mm; the thinner needles are also available in 1/4-size increments.

We knitters need a set of double-pointed needles to knit in the round for small projects. For example, socks, wrist warmers, cuffs, possibly also necklines, hats...

Usually, when knitting in the round, you divide the stitches among 4 needles and knit with the 5th needle. However, I’ve increasingly switched to distributing my stitches across just 3 needles and knitting with the 4th needle, I find this much more comfortable. But that’s a matter of personal preference.

Two needles from a set of double-pointed needles are very helpful when you want to knit something small in rows—that is, flat—or for an I-cord. I also use these needles when I knit cable patterns—as cable needles.

Five needles seem to be most common in German-speaking countries and Central Europe. In other countries, there might sometimes be only four. You then distribute the round across three needles and knit with the fourth.
Another variation involves three short, curved needles for socks, gloves, and other items with a very small diameter. In this case, the two halves of the round are distributed across two curved needles, and you knit with the third. This is good for people who find eight or ten needle tips simply too many to handle. 
I also have a set of double-pointed needles with six different sizes; each set contains six needles. 

There are also very short needles (10 cm) that you can use, for example, for the fingers on gloves. I don’t like using those, though, because the tips poke into the palm of my hand.
Of course, sets of double-pointed needles also come in a wide variety of materials. You’ll have to try them out to see which ones work best for you. For thin sizes, I prefer metal because I can knit quickly with them and it doesn’t break as easily. For thick sizes, wood or plastic works well too—they’re lighter.

Even though I get along just fine with a 5-piece set of double-pointed needles, I’m currently enjoying knitting with a 3-piece set of double-pointed needles.
The needles in the middle are flexible.

While I’m worried that one or more needles might slip out when I’m on the go with a regular set of double-pointed needles, with the flexible needles I can simply slide the stitches into the middle.