As discussed earlier, a maximum limit is specified for the number of httpd processes in the MaxClients directive. However, Apache doesn’t immediately deny requests after this limit is reached. Instead, it maintains a queue that holds the requests until one of the processes is free to accept a request. There is a limit on the requests that can be kept in the queue; you specify this limit in the ListenBacklog directive. When the maximum limit of the requests specified in the ListenBacklog directive is reached, no more requests are accepted, and the 503 (Service Unavailable) response code is sent to the client. The default value set for the ListenBacklog directive is 511.
You will seldom need to change the default value specified in this directive. However, if the backlogs frequently reach the limit, reduce the number of requests specified in the directive; having a high number of requests in the backlog is not good for Apache’s performance