Quantity as result of a formula. Replace the "Quantity" label

This isn't really a feature request, but a support request which aren't answered from this queue. The Commerce Slack channel or even Stackexchange would be a better fit for this.
Besides, updating the label might not be your only challenge, if that is your only "challenge", you can do this via a hook_form_alter() or even just alter the base field definition to alter the quantity field label globally (but again, there are probably other things you have to figure out before that.