Adjusting shipment quantities in Seller Central.
Amazon allows you to edit the units of your shipment’s existing MSKUs up to a 5% threshold above and below the original amount (or up to 6 units). However, you will be unable to add additional MSKUs.
If you are needing to add more units than the above mentioned allowance, you must duplicate the shipment or create a new shipment. Making quantity changes within this threshold will not impact your seller account or ability to send FBA shipments.
Here is a great Seller Central conversation thread about this: