How to create Inventory form?

Do you have anything that I could supply with my customers that shows what I have on hand for quantity? And after they place the order it will subtract from the stock. 

And as I get new inventory and I can always change those numbers..

Yes it is possible to achieve your requirement by adding any of the following widgets:

Gift Registry

Quantity Gift Registry

Inventory

You can add the products in these widgets and set the available quantity of the products. Once a user select a product and submit form, the available quantity will reduce automatically for next user.

Hope this helps.

Do try it out and get back to us if you have any questions.