You can set case quantity per item, and have your FBA Replenish Suggestions adjusted up or down, based on your case rounding settings.
To set your case rounding settings, go to Settings>System Settings>Forecasting, scroll down to "Enforce Case Qty" and choose your Case Rounding Settings. Press “Save” when done.
Now start adding your case qty per SKU. You can do that individually, or in bulk.
Individually – In Shipping>Replenish scroll to the right, and enter the respective case qty.
In bulk – In Shipping>Replenish>Select Action>Update Bulk, download the template and fill out columns “SKU” and “Case qty”, upload it, and refresh your webpage.
Your replenishment suggestions will now be rounding accordingly.