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"?
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.
Just as NS.X said in comment, your translation is already clear
我只喜歡你一個
I like you alone
Or we can translate as follow
You are the only one I love / like.