Sorting by word count needs the count computed, sorted on, and then discarded — the decorate-sort-undecorate pattern.
The pipeline
awk '{print NF"\t"$0}' | sort -n | cut -f2- prefixes each line with its field count, sorts numerically, then strips the prefix. The same shape works for any derived key.
Numeric sort is mandatory
Without -n the prefixes sort as text and 10 precedes 2. The output still looks ordered at a glance, which is what makes this bug survive review.
Ties need a secondary key
Thousands of lines share a word count, and their relative order among equals is arbitrary unless specified. sort -k1,1n -k2 sorts by count then alphabetically within each count, producing a deterministic result. Without it, two runs over the same data can differ — which is confusing when someone tries to reproduce your output.
Delimiter collision
If lines can contain tabs, a tab-delimited prefix breaks cut. Choose a delimiter absent from the data, or use awk to strip the prefix by field position instead.
Try it: Sort by Word Count (Fewest First) on SeoWolf's Notepad.