The attachment file is the initial guide on how to use manageWP. The free version as indicated you can add hundreds of websites to manage in manageWP. However we do not know the limit, but I am pretty sure it’s not unlimited but rather based on your account usage that if you put too much stress on their server they might have the right to disable your account or at least let you know to fix or pay for the service.
manageWP can be trusted similar to google gmail and microsoft live account. Their server is amazon web services also know as AWS.