If you have reached a check-in limit, you will not be able to check in at that partner. The app will show you an error message when you scan the QR code and will not let you enter. On the first of each month your limits will be reset. You can, of course, visit other partners until this time if you have an M, L or XL membership. You can check how many times you’ve been to a partner under “check ins” in the app.
If you are located in Germany don't forget that you can only check-in once a day.