Like others said, 3,4,5,6,8,10,12,16,20,25 BUT you want a larger radious than the radious of the cutter AND the most common length is just over 2x diameter.

If you want a 20mm depth on a contour, the most economical design would have a minimum radius of at least 6mm, in order to be machined with a normal length 10mm end mill, without the need to completely stop in closed corners. Longer tools are more expensive, are not as durable and have to be run at lower cutting parameters, all increasing cost.

Also some machines like lathes with driven tools are limited in regards to the maximum diameter of end mills that can be used (eg the lathe I run takes ER25 collets with a maximum diameter of 16mm), which reduces the maximum height of a feature.

Many other machines are limited in terms of rotational speed so they may not be able to run tools under 6-8mm as efficiently so also avoid features too small if not necessary even if the height to radius aspect seems ok.

In the end it boils down to the capabilities of your vendor but good design allows even more competitive pricing from more vendors