SHELL

number of files in a folder

ls | wc -l #Count files on current directory
ls path/to/dir | wc -l #Count files on specific directory path

number all files in a folder

a=1
for i in *.jpg; do
  new=$(printf "%04d.jpg" "$a") #04 pad to length of 4
  mv -i -- "$i" "$new"
  let a=a+1
done

number all files in a folder

ls -v | cat -n | while read n f; do mv -n "$f" "$n.ext"; done