Apple Push Notification Limits - ios

Question

I'm developing an iOS application that uses push notifications, I have implemented the app and the server side and it works great if I send just one or two notifications. The problem comes when I need to send the same notification to all of my users, the notifications only get to the first users of the loop.
I'm in sandbox, so I wonder if there is any limit for sandbox environment, because I have read that the APNS service has no limit.
Any idea?
Thanks in advance,
UPDATED SOLUTION:
I had to check apple response, I was sending push to invalid tokens and Apple disconnected me from server. With the following function I have solved the problem.
Thanks #Eran and this post
/* FUNCTION to check if there is an error response from Apple
* Returns TRUE if there was and FALSE if there was not
*/
public function checkAppleErrorResponse($fp) {
//byte1=always 8, byte2=StatusCode, bytes3,4,5,6=identifier(rowID).
// Should return nothing if OK.
//NOTE: Make sure you set stream_set_blocking($fp, 0) or else fread will pause your script and wait
// forever when there is no response to be sent.
$apple_error_response = fread($fp, 6);
if ($apple_error_response) {
// unpack the error response (first byte 'command" should always be 8)
$error_response = unpack('Ccommand/Cstatus_code/Nidentifier', $apple_error_response);
if ($error_response['status_code'] == '0') {
$error_response['status_code'] = '0-No errors encountered';
} else if ($error_response['status_code'] == '1') {
$error_response['status_code'] = '1-Processing error';
} else if ($error_response['status_code'] == '2') {
$error_response['status_code'] = '2-Missing device token';
} else if ($error_response['status_code'] == '3') {
$error_response['status_code'] = '3-Missing topic';
} else if ($error_response['status_code'] == '4') {
$error_response['status_code'] = '4-Missing payload';
} else if ($error_response['status_code'] == '5') {
$error_response['status_code'] = '5-Invalid token size';
} else if ($error_response['status_code'] == '6') {
$error_response['status_code'] = '6-Invalid topic size';
} else if ($error_response['status_code'] == '7') {
$error_response['status_code'] = '7-Invalid payload size';
} else if ($error_response['status_code'] == '8') {
$error_response['status_code'] = '8-Invalid token';
} else if ($error_response['status_code'] == '255') {
$error_response['status_code'] = '255-None (unknown)';
} else {
$error_response['status_code'] = $error_response['status_code'].'-Not listed';
}
echo '<br><b>+ + + + + + ERROR</b> Response Command:<b>' . $error_response['command'] . '</b> Identifier:<b>' . $error_response['identifier'] . '</b> Status:<b>' . $error_response['status_code'] . '</b><br>';
echo 'Identifier is the rowID (index) in the database that caused the problem, and Apple will disconnect you from server. To continue sending Push Notifications, just start at the next rowID after this Identifier.<br>';
return true;
}
return false;
}

Answer 1

The likely problem is that some of the device tokens you are using are invalid (remember that production device tokens are invalid in sandbox environment and vica versa). Sending a notification to an invalid device token will close your socket to the APN servers. All the notifications written to that socket after the invalid one will be discarded until you open a new socket.
You can try to read error responses from Apple to find out which device token is invalid.
You should definitely read the error checking section of the Tech Note that was already mentioned by other people here.

Answer 2

There is no limit on the number of users you can send to, you just have to make sure that the size of the message you send it below that limit, which as Kirti stated, is around 2048 bytes.
There is also no limit to how frequently you can send messages, but I would not recommend sending things too often.

Answer 3

Might want to check this out:
There are no caps or batch size limits for using APNs. The iOS 6.1
press release stated that APNs has sent over 4 trillion push
notifications since it was established. It was announced at WWDC
2012 that APNs is sending 7 billion notifications daily.
If you're seeing throughput lower than 9,000 notifications per
second, your server might benefit from improved error handling
logic.