Finder/Terminal: Find files that contain less than 21 lines of text
In Terminal you can combine find and wc for this:

find /path/to/directory -type f \
    -exec bash -c '[[ $(wc -l &lt; "$1") -lt 21 ]]' _ {} \; -print


This will find all files (-type f) beneath /path/to/directory, count the lines (wc -l &lt; "{}", {} gets replaced by any file found) and print the file name for files containing less than 21 lines. 

PS: It will also try to count the lines in non-text files. If this causes issues, use the following instead:

find /path/to/directory -type f -exec bash -c 'file -b "$1" | grep -q text' _ {} \; \
    -exec bash -c '[[ $(wc -l &lt; "$1") -lt 21 ]]' _ {} \; -print


PPS: To start from the current directory, replace the path at the beginning with . (a single dot, for the current directory)

PPPS: To restrict the search to the current directory, use find . -maxdepth 1 -type f ...