You don’t need to put it in your rules.
You can just put it in the pricing section under “extra guest fee”. If your max is 6 guests then put $50/night for each guest over 6. I think it’s a good idea for every host to have an extra guest fee listed for any guest over their max number of guests, regardless of whether or not they charge per guest up to their max.
We have always (4 years now) charged per guest. The base price is for one and then each additional guest has an additional fee. In a community like Airbnb (or at least the community it strives to be), I think it’s the only fair way to price.
For me, it’s not about cost. I give my guests as many towels as they want (and anything else really), I offer them fresh towels every few days anyway. It is about accommodating the most guests, especially solo travelers, because I’ve always been an avid solo traveler. Hosts like to say that it “costs the same” but that’s only for the host, it doesn’t cost the same for a single traveler because they aren’t splitting the price with anyone.
Some hosts will say that it shouldn’t cost more for 2 guests than for 1 if there’s a queen bed (or a 2nd bedroom) but I say that it shouldn’t cost the same. The argument seems to veer to why someone would charge more for the second person, but I want to know why one person should pay the price for two. It leans toward discriminatory. I’m not the only one thinking this way, because a major hotel chain in our city has started giving a lower price for a single guest vs two guests for their rooms regardless the size.
We’ve never had an issue or anyone who was upset by our pricing structure. I tell them the truth, that one person traveling alone gets a discount and two people traveling instead of 3 people, get a discount and so on. People like the idea.
The only thing that comes up is that if someone books for only 1 and doesn’t say, “it’s only me”, then you need to ask them if it’s only them and then go from there (explain your pricing structure and send them a reservation change request). But, from what I understand, hosts who have insurance also have to verify the number of guests anyway.
The only time that we’ve ever had to charge an extra guest fee that was not included in the original booking is when this guy had a baby shower for a small group in one of our apartments. I was a new host and accordingly a little uptight so it really bothered me. I called Airbnb and they said that I could either have the guests leave without a refund or just charge them the extra guest fee that I already had in my price structure. Interesting note, the extra guest fee is charged per check-in day (not night), i.e. the baby shower guests arrived around 1 pm and left around 4 pm. We had a 3 pm check-in time so Airbnb charged them for extra guests for two days since they were there before and after a check-in time. It all worked out, the guy paid and apologized and left a 5-star review, but I am more relaxed now and probably wouldn’t say anything (because he was a decent person, not because he was a guest).
!” (so their’s was a running toilet, but you get the point 
)
. I don’t want people to be able to book for more than my max but I do want Air to know what I charge if they overcrowd. That’s why I have the penalty fee everywhere I can put it. On the rare occasion, I will allow 7 but I charge $30/night, and it’s dependent upon how much activity we’ve had before/have scheduled after and the guest (reviews, did they ask before booking, etc.).