Oh thank you, this is confusing.
I selected manually in the collection, I added the number in stock in the product page so when it's down to '0' it should display 'Out of stock' - I cannot predict some are in stock from my suppliers.
Why do Shopify have to change things when things were simple in the product page. I can add '0' anytime as it is subject to availability.
I have sorted, go to Theme settings > Product Grid > 'Enable' Back in stock request form > Notify me when this product is available