If you are having trouble figuring out the inventory option set then this post is for you.
Inventory Option comes from the WP Travel Pro Utilities module. There are different options under inventory options.
Enable Trip inventory:

This option is especially for tracking the number of seats being booked for a trip i.e if you book the trip then enabling this option will display the remaining seat.
Count Paid Booking for inventory:

Enabling this option will only allow counting inventory for paid bookings whereas disabling this option will count inventory for booking only along with booking with the payment.
OUT of STOCK Options:

This option also allows you to display the Soldout message or enquiry option if the seat is fully booked.
Sold out message:

You can customize messages for unavailable trips.
Note: This option works for a fixed departure as well as in the case of trip duration.
Also, since you can add multiple pricing for the same trip, you can have the seat count separately for a separate pricing option.
For more detail regarding the inventory management, please refer top the official documentation.
For more details, please watch given below YouTube video:
For more details regarding how to set up the inventory option for trip duration, please watch the Youtube video:
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