Unless you have been set up on Amazon with the "Inventory Placement Service", inbound shipments will often be split into many shipments to multiple Amazon Fulfillment Centers. This can happen even with units of the same SKU. For example, if you are creating a shipment of 100 SKU123, Amazon may decide to send 50 to one center and 50 to another center.
However, if the shipment is created with a case quantity, Amazon will not split the quantities per case. In the example above, if you were to define the shipment with a case quantity of 100, Amazon will not break up the shipment and all 100 units will be directed to a single fulfillment center. If you were to define the case quantity as 25, that means you have 4 cases, so Amazon may direct the shipment to 4 separate fulfillment centers. they will not, however, direct it to more than 4 centers.
Configuring case quantity for FBA Shipments