The maximum number of messages to receive per request. Amazon SQS never returns more messages than this value but may return fewer. Values can be from 1 to 10. Default is 10.

All of the messages are not necessarily returned.

Larger values can reduce the number of requests that need to be sent if multiple messages are available. It is recommended to use the maximum value. Reduce the value only if you experience issues and are certain that a smaller number fixes them.

See also: API ReceiveMessage
See also: Amazon SQS – Long Polling and Request Batching / Client-Side Buffering