Setting up the correct number of the maximum number of persons you allow in your property is paramount and ensures a seamless synchronization across all your channels.
Fundamentally, this number must be consistent between FantasticStay and all your connected portals.
To set it up, enter your listing, click Marketing on the left, then select the Amenities tab on top:
In this tab make sure you set up a correct number of guests (which, as commented above, must be consistent across all your channels).
N.B. It is also very important that your extra person fee (should you have any) is consistent with the capacity set up in your listing. For instance, if your listing has a maximum capacity of 6 people and you wish to use an extra person fee, you cannot have a value of 6+ in the "Guests included in the price" in the Channel settings where you set up your extra person fee (Channels > Channels Settings).
Since you don't allow more than 6 guests that value must be 5 or less.
Practical examples below.