10,000 audience

Welcome to the Zoom Community, @alanchow.

The term "broadcast" can mean several things.  If you are asking about how many people can attend a Zoom Webinar, 10,000 is the limit of what you can License directly from the Zoom pricing page (https://zoom.us/pricing/events).

Click the Pay Monthly if you need this short-term, and click the 10,000 for number of attendees.  For more than 10,000 attendees, click the 10,000+ item and you'll see that you need to contact Zoom Sales for more information.

The good news is that this is a simple webinar, and other than having some experience running any size webinar, there's no special equipment or software required. 

Note that you can also stream your Zoom content to a variety of online venues, including YouTube, Facebook, Workplace, or Twitch, but also to any other streaming service that will accept a custom streaming key.  See the articles on this Zoom Support page for additional information on streaming as an option.  You could have a much small Zoom meeting, and still stream to many people through a streaming service.

https://support.zoom.us/hc/en-us/sections/4415050809485-Livestreaming-Meetings