You can earn titles by playing story mode, survival runs, online gameplay etc.
A lot of titles will unlock in the store and need to be bought. You'll also earn additional titles per character with the completion of a survival run and/or online gameplay.
The ones you buy will add to the total titles you've unlocked, but the trophy will only pop at an earned title. I had bought 12 titles yet no trophy. Completed a survival run and the trophy instantly popped for me. Easy survival runs will suffice for the trophy.