Hello @makirpa333gmail-com,
Using the WP travel plugin you can add as many trips as you like from Admin Panel > trips.
But in case of post filter section that is in the Travel log theme, you can only display 9 trips from the plugin altogether.
Also, as much as we would love to help you this we are unable to do as it requires higher code customization to increase the no of trips to display in this section.
So to make more no of trips display you need a code customization and for this, you can hire a proffesioinal developer.
To hire a developer please refer to below given link:
http://wensolutions.com/hire-wordpress-customizer
Hope this helps.
Best Regards!!