Limit number of notification send to a same device

Hi everyone,

Due to change on iOS 13, my chatting app have to use Notification Service Extension to receive message. Now I have a problem that I cant understand. When testing, some device seem to be blocked receiving notification in a few hours (sometime even a day).

I have add apns-push-type to header (recommend from here) but it doesn't work too. In this link, Apple said that:

Verify that you aren't spamming the device. If you send too many notifications to the same device within a short timespan, APNs may treat it as a denial-of-service attack and temporarily block your server from sending notifications.

So I want to know the limit here, when a device is blocked (after how many notification in how many minute?), and if it is blocked, when will it be unblocked to continue receving notification?