Once you face code:3000 then please try to request /webinars/{webinarId}/registrants and compare your limit and registrants count and calculate if is there any registrant limit available or not.
@freelancer.nak We have limit of 500 attendees but not on number of registrants and also when creating webinar we are giving registrants_restrict_number = 0 so that there is no limit(max - 20000 as per zoom) in number of registrations via API. Also we are unable to register in webinar having 0 registrants.