The Communications API enforces the standard notification message sending limits. These limits are per-project and are shared by all of a project's API clients. If these limits are exceeded, an HTTP 429 Too Many Requests error is returned with a time to retry.
Send message limits
To send messages, the rate limit is 3000 quota units per project per minute with a moving average.
List users limits
To list users, the rate limit is 1200 quota units per project per minute with a moving average.