As soon as a person register for your webinar, send them a thank you mail and a confirmation mail. Sending this is not only good manners, but will also help get a confirmation from the registrant side.
Traffic or the number of attendees for the webinar should be near or the equal to the limit of attendees allowed for the webinar. For getting targeted traffic towards your webinar, follow these steps:
Send them invites for your webinars or whitepapers. Send an email regarding it, and wait for their response on it.
Tips to make your webinar more successful