(per month per user *)
* You only need accounts for users who need to set up and administers the Alerts.
The vast majority of customers only use one account, even if the alerts in that account end up emailing hundreds of different users.
(This is a key difference to SharePoint’s built in “Alert Me” emails. With those the user themselves set them up but with Reminder 365 a site owner or similar sets them up for the whole site).