At most 20 BASIS_DIRS args may be specified: Looking to understand the reasoning

[tmknight]

In perusing the code, I couldn't find any documentation/comment explaining the reason for the limitation, only that is was set at 20. For example this limitation will reveal itself if more than 20 --compare-dest args are passed:
rsync: error: at most 20 --compare-dest args may be specified
rsync error: syntax or usage error (code 1) at main.c(1795) [client=3.2.7]

I can work around it, but would like to better understand the technical reason for the max being 20 (i.e. why is there a limit and why is it 20).

TIA