The total content storage on MYOB Atlas including all stock items, images, and videos is 5Gb. Therefore, as long as your total storage is less than 5Gb, you can add more stock items. Regarding alteration of stock levels, AccountRight is not integrated with MYOB Atlas, therefore, the stock will not be automatically reduced when items are sold via the website. You will have to manually do that in AccountRight.
Let us know if you have additional questions on this.