I'd use "units" for what it seems you have there. I'm assuming (like Sue) this is for business.
"items" actually sounds like different products to me. Maybe some variation on a single product or different products. "A company that produces 20 million items" sounds like a few different products are produced, totaling 20 million.
@Tanialoves - sorry if we're causing more uncertainty with all of these different view points.
