I
You have more pens than I (have).
have可以省略,所以就写I.
以下供参考：
用than引导的比较状语从句
It’s easier than I thought.这比我想像的要容易.
She sings worse than she did before.她比过去唱得糟.
She speaks better English than I （do）.她的英语说得比我好.
It is raining worse than it did yesterday.今天雨下得比昨天更大了.
The weather was worse than I had expected.天气比我预料的坏(糟).
I can throw the ball farther than you (can).我可以把球扔得比你更远.
She is now happier than she has ever been.她现在比过去任何时候更快活.