@Mampiti0 yes, you will be penalized if *you* cancel the booking.
That is why, if you want to cancel, please contact Airbnb support and explain the situation. Then ask Airbnb to either help contact the guest or to cancel the booking without penalty.
Before you do that, though, please send a very clear message to the guest, saying that they need to confirm the number of guests. Explain that currently the reservation is for only one person, so you will only allow that one specific person to stay in your property. Anyone else will be forbidden from entering.
If they want to have more than that single person stay with you, they need to tell you in advance how many are coming and who they are. Also tell them the price will be adjusted for the number of guests.
Finally, inform them that if they do not respond in some way that you may cancel the reservation.
*Then* contact Airbnb (if they do not respond).