Passing Variable In Onclick?

Basically, I have a form with some text fields, and I want to put a button next to each text field, so that when a user clicks on the button, it increments the value in the field by 1. I know how to do it if I know the id of the text field, but what if the id of the text field is a variable? How do I do it? Here's my code:

<script language=javascript>
function process(v){
document.getElementById('order_item' + v + '_quantity').value++;

[code]....

My issue is with my onclick statement:

<input type="button" value="Add +" onclick="javascript:process(0)">

This works fine for my first field, but I need to have one for every text field, so how do I pass my counter variable (which determines the id of the appropriate text field) to the process function?