Magento provides you with an easy and yet powerful way to manage your product quantities. To do this, first login you Magento admin area and go to Catalog -> Products.
On this page, you will see a list of all of the products you've added to your Magento store. Locate the one you want to edit the quantity for and click on it to edit it.
Doing so will take you to the page where you can edit every aspect of this product. Scroll down to the Quantity section and add the available amount of this product.
That's it, you can now track number of products of this type that you have in stock.