Zoom Webinar - webinar size

Hi,

I wonder if you can limit the number of people who can attend a webinar. 

I have just set up a test Zoom webinar and can see that the webinar size is 500.  I would like to set up several webinars and then send out the Registration Link to everyone so that people can register but would like to limit the number of attendees to 20 for each webinar - therefore if the number of 20 people has been reached on a webinar and someone else tries to register they will be advised that this webinar has reached its maximum numbers and they will need to register for an alternative date/time. 

Is this possible or is the number set at 500 and can't be changed?

Many Thanks

Mandy