18-135mm is not a prime lens. A prime lens has a fixed focal length.
The focal length itself depends highly on your taste but in any case you should buy a lens with a wide aperture (low f-number). This makes it easier to get blurry backgrounds.
I used to love taking portraits at 200-300mm (on a crop sensor like yours) which often resulted in images where the subject didn't even notice you, which is the most natural expression you can get.
Now I'd recommend a 70-200mm/f2.8, which is expensive but you get a nice background blur and the quality is really good.
You will need 35mm or 50mm if you take a picture of the whole body or group photos and not only the face. Plus, you get more context if you use wider angles, because the viewer will understand what the scenery was and not only see a face with a blurry background.
I would not recommend 35mm AND 50mm. Buy one of those and use your feet.