for i in *.txt; do
new=$(printf "%01d.txt" "$a")
mv -i -- "$i" "$new"
let a=a+1
done
# "%01d.txt" 01 is the number of digits you want your name to be composed of,
# Example: %01.txt --> 1.txt , 2.txt, 3.txt, ..., 10.txt, ...
# %04.txt --> 0001.txt, 0002.txt, ..., 0010.txt, ...