If I were to translate "I only like you" I'd say
Wǒ zhǐ xǐ huan nǐ
However I have seen the following
Wǒ zhǐ xǐ huan nǐ yī ge
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.
Just as NS.X said in comment, your translation is already clear
Or we can translate as follow