1. Head to the Product Library or search box for the product you want to have a set inventory on.
2. Make sure the "inventory limit" switch is turned on (marked blue)
3. Enter the number value into the limit box that is your inventory limit.

   

4. Click the save button in the bottom of your dashboard 

Keep in mind: Once the product has sold out, for it to be made available again you will either need to set a positive value on the quantity limit or turn off the inventory limit entirely.