The limit is actually 10K as per the Webinar reference doc: https://developer.webex.com/docs/api/guides/webinar-guide
Developers should note that registration capacity has a maximum limit of 10K. There's no minimum limit, which means that a webinar can support any number of invitees or registrations fewer than 10K.
Do you mean you want to allow more that 10K invitees? If yes, please note 10K is the max limit allowed.
Best regards,
Rudrakshi Srivastava