In “我只喜歡你一個” what does the "一個" do? If I were to translate "I only like you" I'd say


  我只喜歡你
  
  Wǒ zhǐ xǐ huan nǐ


However I have seen the following


  我只喜歡你一個
  
  Wǒ zhǐ xǐ huan nǐ yī ge


Translated as


  I like you alone


What is the effect of adding the "一個"?
It is there for emphasis.

The two sentences have similar meanings, while the one with "一個" has more emphasis on that you are the ONLY one that I like. It can be used to transfer stronger emotion.