To limit the quantity you wish to list, use an Inventory Rule.
Inventory rules are located on the Channel Settings page within each of your connected channels, and allow you to set the quantities that are applied to a product when listed.
You can choose to only list a percentage of your available inventory, a maximum quantity, or a minimum quantity. The rule will be applied to any new or existing listing for that channel.
Rules also control how much inventory is sync'd to your channel.
If you set a maximum of 2 units, every time we sync your inventory (which happens every 15 minutes), we will limit the quantity on that listing to 2. Likewise if you set a minimum, we will never show you have less than that minimum on your listing.
A minimum is recommended if you always keep extra stock of an item or never have to worry about running out.