I find it easier to charge extra above 4 people. I found that if I didn't and my maximum number was set to 8, the only inquiries I was getting was for larger groups. Too often they were for more 10-12 people and I don't want to rent to groups that large.
We have the room for up to 8 guests, but it's really more comfortable for me to host 4-6 guests. Larger groups are just too hard on my house and I'm on pins and needles waiting to check the condition of the house after they leave.
My rental agreement asks for the names of all the guests (which I like to have in case of an emergency and need to know who was staying there) and to check the number of guests coincide with the number confirmed.
I also have a clause that any guests who stay overnight unexpectedly will be charged as an additional guest and that visitors are to be pre-approved.
If AirBnb won't look out for me as a host, I will.