I don't know why you have limited titles. I'm running older iMovie version 10.1.12 on Mojave, and my app shows 48 titles.
Try deleting preferences and see if that cures it. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon. iMovie will open in that library.) Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If deleting preferences doesn't help, try redownloading iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your account/purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.
-- Rich
