Hi
- You can send any number of messages, but according to current throttling quotas for Basic SKU, you will be throttled if send more than 100k messages per minute. Standard SKU has 10 times bigger limit of 1M per minute
- You need to check the notification outcome. When you perform send, you get back an URI where you can check the result of asynchronous send operation. But details about expired channels are only available for Standard SKU. Maybe I missed something, you can just check yourself, query that notification outcome URI and see what it says