@Susan1803 Your house is extremely inexpensive, even with the extra person fee added for 6 people. If you want to avoid this headache in the future, you may consider just having a flat rate whether it's 1 or 6 people (I would still book for two at your 6 guest rate.)
Or in the future, you could also let the guest know when they book that the number of guests must be accurate. Say something like "The Airbnb booking system has your reservation confirmed for X amount of guests. If this number is incorrect or changes before your arrival, please send a message ahead of time to let me know. It is important that we know how many guests will be staying to better prepare the space and charge accordingly.
Also, put in your house rules that there is a per person charge of X amount for each guest over two. This way the guest can't say that they weren't made aware.
As for your current reservation, you could send the guest a request for more funds for the extra person. Click "Send or Request Money." But weigh the costs of potentially getting a bad review or the guest playing dumb and refusing to pay.