@NIJJER, Harpal (OXLEAS NHS FOUNDATION TRUST) ,
It has no specification about this question in official documents.
According to my understanding, the limit should inherit Teams meetings maximum limit – 350. For more information, you could refer to this link.
Meanwhile, I do a test in my lab. It has 8 participants in a Teams meeting. When I create a new breakout room, it shows that all participants could be assigned to this room. Due to the limit users in my test lab, it cannot reach up to the limit 350. If your organization has adequate users, it recommends to verify the guess.
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