To switch the 'Stock Quantity' feature on/off, from the store dashboard go to:
> Store Settings > Products > Show/Hide

From within this page, the top tick box labelled, 'Stock Quantity' will either enable or disable the feature.
You are also able to enable/disable the 'Stock Quantity' based on the criteria you set below. E.g if you check the tick box and set the number to 'Only if up to 10 remaining', then the 'Stock Quantity' will only enable if this criteria is matched and there are 10 or less of that product in stock.

(*Don't forget to click 'Save Changes' after making alterations*)