If you are want Listing Mirror to hold back X amount of quantity in reserve, you will want to take advantage of the Holdback Quantity feature.
You can find this setting on the edit integrations page, under the inventory tab.
Enter the amount of quantity that you want Listing Mirror to hold back from the marketplace and then click save integration.
Note, if you set your holdback quantity to 3, and you have only 3 quantity left, Listing Mirror will send 0 quantity to the marketplace.
Note for bundles and kits: holdback quantity will work at the listing level not the component level with bundles and kits, meaning we will subtract the holdback quantity from what the total would have been after calculating the the kit inventory.