Hey @v.hristov,
As this is more of a question about the functionality of Webinars and not about the API or App Marketplace, I suggest you reach out to support.zoom.us for more information. As of now we have no plans to limit registrants.
That being said, you could literally have 10000 registrants, however, only 100 would be able to join the webinar. But, that 100 includes the host and the panelist. So say they have 5 panels and one host for the webinar, that only leaves a spot for 96 participants to join the webinar. If someone happens to drop out of the webinar and it goes down to 99 then the next person waiting to get into the webinar will enter if they are outside of the webinar.
Thanks,
Tommy