Retrieve multiple messages from SQS

There is a comprehensive explanation for this (arguably rather idiosyncratic) behaviour in the SQS reference documentation.

SQS stores copies of messages on multiple servers and receive message requests are made to these servers with one of two possible strategies,

• Short Polling : The default behaviour, only a subset of the servers (based on a weighted random distribution) are queried.
• Long Polling : Enabled by setting the WaitTimeSeconds attribute to a non-zero value, all of the servers are queried.

In practice, for my limited tests, I always seem to get one message with short polling just as you did.

AWS API Reference Guide: Query/QueryReceiveMessage

Due to the distributed nature of the queue, a weighted random set of machines is sampled on a ReceiveMessage call. That means only the messages on the sampled machines are returned. If the number of messages in the queue is small (less than 1000), it is likely you will get fewer messages than you requested per ReceiveMessage call. If the number of messages in the queue is extremely small, you might not receive any messages in a particular ReceiveMessage response; in which case you should repeat the request.

and

MaxNumberOfMessages: Maximum number of messages to return. SQS never returns more messages than this value but might return fewer.