This is a parameter that can be easily modified.
To set the default number of nights displayed when making a reservation, follow these steps:
Property Settings > General > Property > Preferences
Within the property preferences, scroll down until you find the "Reservation Forecast Settings" section.
Within this section, you will find the "Average length of stay" field.
Here you should enter the number of nights you want to appear when searching for availability for a new reservation.
Once you have entered the number of nights in this field, it will be set as the default setting for the length of stay when making a reservation.
In this way, every time you search for availability, the number of nights you have configured will automatically be displayed.