Incremental number in shell script on each loop

Question

#!/bin/bash

echo SCRIPT: $0
echo "Enter Customer Order Ref (e.g. 100018)"
read P_CUST_ORDER_REF
echo "Enter DU Id (e.g. 100018)"
read P_DU_ID

P_ORDER_ID=${P_CUST_ORDER_REF}${P_DU_ID}


#Loop through all XML files in the current directory
for f in *.xml
do
  #Increment P_CUST_ORDER_REF here
done

Inside the for loop how can i increment P_CUST_ORDER_REF by 1 every time it loops

so it READs 10000028 uses it on first loop
2nd 10000029
3rd 10000030
4th 10000031

Answer 1

((P_CUST_ORDER_REF+=1))

or

let P_CUST_ORDER_REF+=1