[kh99]

The setting is Settings > Options > Email Options > Number of Emails to Send Per Batch. You might want to increase that until the queue is gone, then maybe decrease it and watch to make sure your queue is getting processed as needed. It might not matter if you leave it at a high value except that ideally you'd want the sending to be divided into smaller batches.

It seems that the "send email to users" function is confusing since it actually queues the emails (if you have the email options set to use the queue), so "email to send at once" doesn't make much sense.