#!/bin/ksh

...possible code...

# Declare inputs as integers
integer n1=0 n2=0

...possible code...

# Take n1 input
until (( $n1 > 0 && $n1 < 21 )); do
  print -n "Please enter a positive integer between 1 and 20: "; read n1
  if (( $n1 < 0 || $n1 > 20 )); then
    print "$n1 is out of range."
  fi
done

# Take n2 input
until (( $n2 > 0 && $n2 < 21 )); do
  print -n "Please enter another positive integer between 1 and 20: "; read n2
  if (( $n2 < 0 || $n2 > 20 )); then
    print "$n2 is out of range."
  fi
done

trap command signal1 signal2 ...