Please note that this is recommended on any products that don't have any product variants.
If you wanted to limit the amount of stock that a particular product has, just follow these easy steps.
Step 1) Under "Manage Store Products", click onto the product you want to limit the stock of.
Step 2) Under the General Tab of your "Edit Product", scroll down to "Stock Control (Optional)".
Step 3) Once you've unticked "Do not limit product availability", you will be able to input the total amount of stock available for a product.