That is a neat question. I think any one shop has a limit and that limit is per shop not per App. So if you decided to do many thousands of API calls you'd be blocking other Apps that wanted to play with a few calls. The reality may differ from that. You'd only know by testing.
Besides that, I doubt you're sitting on a pile of a million Shopify orders. A hundred thousand or so, sure.. maybe even three hundred thousand. A million order shop is probably still a rare bird in Shopify.