Hi,
Sound like you can create a room mailbox and you send invitation to all users with the room mailbox specified. There is a value "capacity" you can set for a room mailbox but it's sole purpose is to show capacity, not enforce it. So it will not prevent the user joining the meeting even the capacity has been reached.
That's all I know from Exchange side.
Maybe you would like to vote in this link: https://outlook.uservoice.com/forums/322590-outlook-for-windows-desktop-application/suggestions/32006899-max-attendee-limit-for-meetings
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