Person Capacity not a selectable option in automation fields list

We need to make it clear to people that we have a maximum number of guest allowed to stay at the property.  Guests = people of all ages.  Our VA consents from the local government stipulate the maximum number of guests and then we have Airbnb that does not count infants in the total.  We tell the guest this when booking on Airbnb but we also want to include it in the message we send to the guests when they make a reservation.

At the moment I have created a custom field on the listing but it should be an option based on what is entered under person capacity in the listing so I would not have to rely on the custom field.