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My first DSLR camera was Canon EOS 350D. Now I've got an upgrade to Nikon D90. Both were/are with the kit lens.
Whenever I'm taking a picture with biggest zoom (55mm on Canon, 105mm on Nikon) the diaphragm setting always stops at f/22. Also, when using aperture priority mode the biggest setting I can set is f/22. Any reason behind this number?
It's just a limit for the construction of the diaphragm. Either the the specific construction makes it impossible to close further, or the manufacturer has chosen that point as the limit to preserve quality.
The diaphragm is usually made out of blades with a curved edge. The curve is formed so that it makes a hole that is as round as possible at the biggest settings. When the hole gets smaller, it goes from being almost round to being almost formed as a polygon. For a six-blade diaphragm for example it would approach the shape of a hexagon. The un-roundness of the hole reduces the image quality somewhat, so you don't want to get too close to the polygon shape.
Also the optical effect called diffraction reduces the image quality when the hole gets too small. When that happens depends mainly on the sensor size and resolution.
The aperture range has nothing to do with your camera. It has to do with the lens.
Each lens has its own aperture range which may be different along the zoom range for a zoom lens. The limit on the wide-end is extremely important as it dictates how much light can come in but puts more requirements on the lens design. Larger maximum apertures increase the cost of a lens significantly.
On the narrow end, the limit is somewhat arbitrary. Most modern lenses stop around F/22 which is actually too much anyway. That is beyond the diffraction limit on modern DSLRs and will cause your entire image to become softer.
