Counting users logins on separate devices

1. Save user id and session data in database after user clicks log in
2. Every page visit, check database on how many connection they are making
2.1. If value exceeded X then logout first session (or how many you want)
3. User visits again and they need to login again due to the session have been destroyed

Thank you for your input and advice,
What I'm concerned with this approach is adding a database call on each page would slow the site down..especially on a busy site.
Also I know how to check if there is a session for user logging in but how do I check if another users session exists in the database? If I can  search in the session database for another users USERID then I could start a count.