Is there a way to sort the customer items by item number under commerce customer orders in the control panel?
There is no current built-in solution for this. I suspect they are displayed in the order they were added to the cart most likely.
Your best hope would be something like the cpjs addon and some magic JS to swap the rows, but it would be...non-trivial at best.
You might want to put in a feature request here: https://github.com/craftcms/commerce/issues