At this time, being able to view your remaining number of products upon entering the item on the invoice isn't available. To see the quantity left for your items, you'll need to go to the item list and check it from there.
Here's a sample reference on whereto view them:
Please know that our developers are constantly finding and considering new features to be added to the system and cope with your business needs. That said, I encourage you to visit our Firm of the Future site to be updated with our latest enhancements and product road-maps.