I've been looking for a specific explanation on how many messages Google queues for a particular offline device before it starts dropping them? I've read that only 4 unique collapse keys can be queued for a device at a time but I just tested sending 5 messages with unique collapse keys and when i brought my device online, I got all 5. I need to know how many messages I can expect Google to hold on to and also how many unique collapse keys I can have queued at once.
Thanks a lot for your answers.
The answer would likely be unhelpful since neither C2DM (now deprecated) or GCM will guarantee message delivery. But in any case the limit for GCM seems to be 100. You can queue upto a 100 messages for delivery without collapsing any of them. Source