If you have no actual functionality besides logging in, you can just use HTML and CSS to create your custom login page - to do so change the login.html file of your theme.
If you have additional functionality available to people before they log in, for example a reset password option, you would use the login form widget and enable anonymous users.
Anonymous users create sessions (otherwise, they would be unable to use your additional functionality) and count towards your concurrent user limit.
You may want to investigate this however: if the total number of concurrent users remains constant, anonymous users would have no impact, since the only thing that changes is the moment their session is created. If users who are logged out are redirected to the log on screen (and now have an anonymous session), or you manually create sessions without removing the anonymous session, then you would see an increase in the number of concurrent users.
Thanks for the info. We do use forgot password functionality, so I don't think an html login page would work.
I just implemented a fix with the MicroflowTimer that redirects users to our public website after a period of time on the login page. The public website has a Login link that takes people to the app. This will prevent us from having users that just have the Login page in a browser tab from counting against our concurrent user limit.