@Elizabeth1411I would just start by asking the guest in a non-confrontational way through Airbnb messenger. Maybe something like, "I think there may have been a mistake on your reservation: you were only booked for one person. It's an easy mistake to make on the app. [Aside: it actually is. I was booking on the weekend and almost accidentally booked for 1 instead of 2]. Can you let me know your group size so I can amend accordingly? Thanks!" Then if they respond with the number, you can send a request for more money, and you have it in writing if you need to get Airbnb involved.
If they ignore it or lie, there isn't much you can do. You can send a request for money regardless, but if they don't confess to it, you don't really have proof. I would still consider calling Airbnb about it if they don't respond. Make sure you document the mess.
I agree with Emil: if you get a booking for one person only, maybe just verify in future. Seems a bit odd.