Hi @Ana40534
Your listing description presently shows 8 guests. You should set this to the maximum that you want to accommodate, otherwise your listing will not come up when someone searches for a group larger than 8 guests. To change this, go to the Listings tab, click the listing and scroll down to "number of guests" (Airbnb says this should be the number of guests that can "fit comfortably in your space"). It sounds like it should be 12 for this listing?
To add an extra guest fee for groups larger than 6, go to the Calendar and click the Pricing tab (in the mobile app you have to click the settings icon at the top to get to Pricing).
Scroll all the way down to Additional Charges. When you click this section, you will see that Extra-guest fee is one of the options you can select. You can choose how much you want to charge, and after how many guests (6 in your case) you want the charge to apply. Hope it helps!