You can invite up to 150 people into your Hangouts chat and 25 in a video call.
Opening a Hangouts Group Chat
1. Open Hangouts in either Gmail or Hangouts web page.
2. Open up a chat with one of the people you want to invite into the group chat.
3. Press the icon at the top of the chat screen.
4. Write the name or the e-mail address of the person you want to invite. Tap the box next to their name and click Add people.