@Shahrzad10. Yes she is breaking your House Rules. You can contact Airbnb and ask them to cancel booking based on this. She will retaliate, and leave a very poor review, which you will then try to get removed and around it goes (based on other cases here on forums).
I know max is 2 but what would you charge as an extra per person per night? I gather you didnt supply exta linen for the sofa bed so this is more about electricity and water usage? How long is the stay?
So yes you can ask for an extra amount via Resolution Centre, which she may pay or refuse, and you still end up with a bad review because she thought she had the right to bring an extra guest.
Moving forward please remove the second picture of where guests can sleep, unless you want to encourage people to sleep on the sofa bed regularly, in which case extra linen must be provided. If you do, you could increase your max capacity to say 4 and increase your nightly rate OR leave rate as a base for 2 and have a clear per person extra fee above that.
Either way I would be updating your House Rules to mention a per person fee if unauthorised extra persons end up arriving. Holds more weight when you go to do the request through Resolution Centre.
All this is my opinion, but when it happened to me and I pointed it out to the guest, they offered to pay but then refused and it all went down hill from there.
So now I have clearer rules and fees. I hope it helps. 🙂