Is there an OOB Inventory module, where user can maintain available stocks, then each time user will create new Order, ordered quantity will be deducted on the available stocks.
Also you may create a folder, where you can store all available orders etc.
Alternatively, you can create some boolean variable, where the values will be available/not available and show this field on the order page.
Another way, you can create int variable with amount like "Available stock". You may show it on the Order page and set default value on the object to this variable (etc. 100 - the available amount). Then set up business process where in Script Task will substract from the formula quantity from this field - if the order was added and so on.
Here some links of how to work with script task and formulas:
