Normally, every scan is checked live
The scanner asks our service whether that ticket is valid and whether the rule you chose allows this entry. That answer is immediate and identical on every phone at your door.
Offline, the phone answers from a copy
You download the guest list to the phone while you still have internet. After that, the phone can recognise tickets on its own, using what it downloaded.
The limit is what other phones did meanwhile
An offline phone cannot know that another phone just accepted the same ticket. If refusing a second entry matters to you, keep one phone per door online, or accept that duplicates are settled when everything syncs again.