@Katharine19 In many instances Katharine a guest will think it doesn't matter how many guests they declare because the host does not set an extra guest fee and so they can bring as many as they like.
This is not your situation because you do set an extra guest fee after one guest, and, while not being a major increase, over 2-3 night stays for a group of four guests it will certainly make a sizeable difference, so you are probably going to find that there will be a percentage of guests who are going to try an chizel a bit off the bill by under declaring guest numbers.
If you strike resistance to this Katharine you can always state in the message stream with the guest that it is really important that guest numbers be declared accurately because Airbnb will only offer their umbrella protection to hostings with declared guests, and as undeclared guests will not be covered this presents a substantial risk, and one you are not prepared to take!
Although technically this is not correct, it sounds logical to the guest and in almost all instances the guest will understand and comply!
I hope this helps you Katharine.
Cheers.....Rob