the typical and classic max is 18 some *newer cgminer mod versions use a RAW method combined with a max of 13.
so i don't think 20 or 21 or higher would be valid and it would default to some other value.. i'm guessing
20 was the old max in cgminer. Up to 23 is valid in the newer branches of cgminer such as sgminer. And any scrypt or nscrypt miner worth his salt should be using sgminer or some other maintained branch by now.
Classic intensity values (when you aren't using RawIntensity or shader-based XIntensity) launch 2^I threads. e.g.
I:13 = 8192 threads
I:14 = 16384 threads
I:15 = 32768 threads
I:16 = 65536 threads
I:17 = 131072 threads
I:18 = 262144 threads
I:19 = 524288 threads
I:20 = 1048576 threads
I:21 = 2,097,152 threads
I:22 = 4,194,304 threads
I:23 = 8,388,608 threads
As you can see, the number of threads increases exponentially with higher values on I. This is why using Xintensity for tuning is so much more effective. Not to mention these values rarely match up with the number of shaders on the card, and the optimal number of threads to launch is a multiple of the number of shaders on the card. This is why I=13 is the most effective on many devices - because they have 8192/N shaders.
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The sgminer git tree even has pool-specific algorithm/nfactor support, and preliminary kernel hotswapping!