Most models fly at 60 - 90 mph wide open. Most glow 2 cycle engines are happy turning around 12,000 rpm range give or take. And so the 6 pitch is the most common for this combination.

A glow 4 cycle revs up a few thousand less, so 7 and 8 pitch is common for those. Larger gas motors turn even slower, typically 6,000 to 7,000 rpm, so 8, 10, and 12 pitch are common.
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Multiflyer, what you say is true in generalization only. "Most" is nowhere near "all", and I'm not sure "most" can even be used as a generalization. In fact, there are so many types of engines, props, planes, and flying styles, I'm not at all sure a generalization like this can be made. Each engine/prop/plane combo is different and has different requirements, depending on how the pilot wants the plane to fly. Have you ever used a radar gun on an R/C model? Most do not fly anywhere near 90 mph. All engines are different and each has a different optimum RPM. The RPM requirements will change from plane to plane. I have a Webra Speed .32 and an OS .32 that are both happy turning 14,000. These engines use 10 x 4 props. I have a Zenoah G-23 that is happy with an 18-6 prop. 4 stroke glow engines can turn larger diameter props with less pitch, and are often used for this purpose.

True, there is usually a good base for prop diameter and pitch to start from. After that, experimentation is the only way to achieve the best combo for a specific plane's use and the pilot's flying style. The pitch and diameter may go way outside your figures, and even the manufacturer's recommendations. Blanket assumptions just can't be made.

Dr.1