You should have at least 4. I think that most kids would enjoy about age 10 or higher
Depending on the scenario you choose 2 or more people would be fine. I'm sure someone somewhere did the one man show (person), but the more the better.
The rooms aren't easy which is what makes them fun! That said, I think you need a minimum of 4 people. (One of the rooms u have to make a human chain. 2 people isn't enough and you wouldn't be able to get out.) Kids as young as 10 can enjoy them but if you want them to be very helpful in figuring out clues 14 or older is better. Good luck!
We do them with either just my wife and I or with our 9 year old.
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