You'd first add all of your 'Accessories' as a normal inventory item with the quantity posted and the daily rates and all those sweet details.
Then, when you go and add this item as an accessory to another item, the quantities will be tracked.
If I have 100 chair covers in stock I'd make those first with quantity and pricing.
Then I'd create my 200 chiavari chairs with their quantity and pricing and add the chair covers as an accessory.
I can specify if the accessory is going to be hidden from the contract and also how many should be added and if it is required or not.
So let's say I put a chair cover as an accessory to my Chiavari chair.
Then when I add my Chair to a project, I will be able to add a chair cover on the project.
I can see how many I've posted and how many are required or suggested
If I over book my accessories it will still show as a conflict on a project in the 'Item Relations' window and on the project, like-a-so: