The way our sync logic works is that we use the lower of Quantity on Hand and Available Quantity.
Quantity on hand would be the physical inventory count that you have in stock
The available quantity is a feature that allows you to mask that number to a lower number on your sales channel. By default, ecomdash sets this value to 999 to always have your quantity on hand synced with your sales channels.
If you want ecomdash to always sync a fixed number other than quantity on hand, check the check box 'Always sync available quantity' and update the available quantity field to a value that you want.