Until now, participants to Teams webinar were able to access the webinar from unlimited devices.
Because registered participants are were/are able to share with others their registration link, this cause discrepancy in attendees report.
Now, access to webinar can only be done from 3 concurrent devices, meaning if registered attendees share their link they may not be able to access the webinar if more than 3 other participants using the same link have already joined the webinar.
It is recommended to communicate with your users to let them know if they want to share the webinar with other they have to share the registration link instead of their invitation.