@Aurora204 Guests can't add extra people to the booking without your approval. So your options would be:
a) Use the Change Booking option to update the guest count, and add any fees (extra person and/or pet) that are already disclosed in your listing. Only grant access to your property after they have accepted the Change request
b) Inform the guest that you can not accommodate their requested additions and ask whether they still want to stay solo or cancel their booking.
I'm not really sure what you think Airbnb is supposed to do for you here - it's your home, and entirely your decision who you let into it.