Google-hangouts – How to schedule a Google Hangout

You can use Hangouts to talk with more than one person at the same time.

• Conversations can include up to 150 people.
• Video calls can include up to 10 (Gmail, G Suite Basic) or 25 (Business, Education) people.

Everyone who wants to join a group conversation must use Hangouts.

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For the desktop environment:

Specifications for sending and receiving messages

You can have a group Hangout with up to 100 people.

Requirements and specifications for video calls

Video calls can have a maximum of 10 participants.

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In general:

Sometimes you want to have a Hangout with multiple people at once, like when you're planning a trip with several friends or choosing a convenient meeting time. With Hangouts, you can have a discussion with up to 100 people and have a video call with up to 10 people.

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Also:

Hangouts video calls are limited to 10 video conference participants, with no time limit. You can also use Hangouts On Air to broadcast a Hangout to many more people, though there is still a limit of 10 active participants.

If you activate Google+ premium features, the limit increases to 15 participants for both Hangouts video calls and Hangouts On Air. The limit remains at 10, however, for Hangouts created from the Google+ Events page.

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You can use https://gchat.simon.codes to set up an automatic reply (disclosure: I made it) . It's intended for unused accounts though, so it might not be what you're looking for if you often log into the account.