Hi,
The stock next to the quantity input field is the quantity of the main product.
Options can have their own stock, but it's not displayed there.
Usually, the stock of options is unlimited (like when you have options for pizza toppings or hardware options for computers)
If you want to display the quantity of each choice of an option, usually, that's done inside the dropdown selector via an override of the view file product / option.php via the menu Display>Views.
For example, this line there is to add the price of each variant of the option product:
$text .= ' ( '.($positive?'+ ':'').$this->currencyHelper->format($price, $ok->price_currency_id).' )';
So you could add a similar line for the stock:
$text .= ' '.$variant->product_quantity;
Now, to swap the stock displayed next to the input field based on the option selected is more complex. It would require some javascript custom code to update the displayed quantity based on what is selected. It's not impossible, but it requires proper development work.