Looking at the Hangouts On Air common questions - and within those FAQs under "Invites and Viewership" there is this question, How many people will be able to watch the broadcast? with this answer:
Your broadcast will be viewable by the public once you click Start broadcast from within the Hangout On Air. The number of viewers watching your broadcast is displayed at the top of the video call window next to the “Embed” link.
Thing is the answer is either vague, or doesn't answer the question; meaning does the "viewable by the public" mean 1000s of people, or those within some unstated maximum.
Just to be clear, I'm not referring to how many people are able to outbound stream from within the hangout, which I believe is limited to ten users - but how many users are able to view a live stream.