Actually, if you have 50 articles and 2 buyers simultaneously purchase each 30 articles, then the shopping cart in this case will not reserve the quantity and will let both users to add such a q-ty of items to cart.
You can use option to specify \"minimum quantity for Item\’s status to be out of stock\”. Lets say you have 50 articles and you set Minimum quantity to 10, so if 5 people decide to buy 10 articles, then the person who will be the last one will not be able to do it, as the system will tell him that there is no enough quantity of articles (even if there are 10 articles and buyer wants to buy 10, but it will not possible as you set minimum q-ty option to 10).
But to tell the truth, the possibility that buyers will be buy great amounts of products simultaneously is not so big.
You can use option \"Notify for quantity below\” and you will be notified if the q-ty is less then you set, you you will be able to re-order the necessary products before you will be running them of stock.
If you need in bulk to set the value for options I mentioned - \"Minimum Qty for Item\’s status to be out of stock\” and \"Notify for quantity below\” for the necessary products, you can use the application called Store Manager for Magento, there you simply select all products and set values for those options.
If you are interested, here is 14-days free version available for download -
Hope it was useful.