A listings always has to have number of guest allowed to stay in. It is defined in field maxGuest.

Listing can offer extra beds which allows more people to stay. In that case, number of additional guests will be specified in additionalGuests.amount.
So actual maximum number of guests allowed on the property will be maxGuest plus with additionalGuests.amount

An example of number of standard guests and additional guests in API Listing:

"maxGuest": 3,
"additionalGuests": {
        "amount": 2,
        "price": 5.85