you can limit available seats for booking of trip using WP Travel pro plugin. WP Travel pro plugin provides the inventory option with which you can limit number of seats for booking of trip.
You can enable inventory option and set the number of seat by going to Admin Panel > Trips > Individual Trip(edited) > Inventory Options so that customer will be able to book the trip only for limited seat.
Note: This feature will be applicable only in case of Fixed Departure not for Trip Duration.
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