The amount is added when the customer does an Add to Cart and states how much they want to add ...
The idea behind a custom page for the product _info either by identifying this 1 product or using a different Product type was so that you can make it "look different" from the other products and customize the Add to Cart button or the way the Price reads such as not show the price at all ...
If this is a $1 product then they enter 500 and now it is a $500 donation ...
You can customize the template to check the products_id and if, for example, this product was products_id 22 you could make that 1 product read differently or you can use a different Product type and make it look differently ...
As to the quantity if you are tracking stock that is done for everything ... you would need to again use the customization not to show the stock ...
If you are not familiar with writing code and customizing you may wish to post in the Commercial Help wanted for help on this ...