c0.pdf   c12.pdf  c15.pdf  c18.pdf  c20.pdf  c4.pdf  c7.pdf
c10.pdf  c13.pdf  c16.pdf  c19.pdf  c2.pdf   c5.pdf  c8.pdf
c11.pdf  c14.pdf  c17.pdf  c1.pdf   c3.pdf   c6.pdf  c9.pdf

gs -q -sPAPERSIZE=a4 -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=out.pdf *.pdf

$ for f in *.pdf; do echo $f; done
c0.pdf
c10.pdf
c11.pdf
c12.pdf
c13.pdf
c14.pdf
c15.pdf
c16.pdf
c17.pdf
c18.pdf
c19.pdf
c1.pdf
c20.pdf
c2.pdf
c3.pdf
c4.pdf
c5.pdf
c6.pdf
c7.pdf
c8.pdf
c9.pdf

gs -q -sPAPERSIZE=a4 -dNOPAUSE -dBATCH -sDEVICE=pdfwrite
   -sOutputFile=out.pdf $(ls | sort -V)

echo *.pdf(n)

FILES="c0.pdf"
for i in $(seq 1 20); do FILES="${FILES} c${i}.pdf"; done
gs [...args...] $FILES

gs [..args..] c{1..20}.pdf

c?.pdf

a?.pdf a??.pdf b?.pdf b??.pdf c?.pdf c??.pdf