I have twice as many pens as yours, is it grammatically correct?

In my opinion, it should be I have twice as many pens as you.

YoyoimutI have twice as many pens as yours,

You should write your sentence this way:
I have twice as many pens as you.

Or this way:
I have twice as many pens as you do.

Thank all of you replied this question.

I understood.

YoyoimutI have twice as many pens as yours, is it grammatically correct?

Thank you

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I have twice as many pens as you [have]

I have twice as many pens as yours, is it grammatically correct?

It is grammatically correct, but you need to realize that it means something very very strange.

In your sentence yours means your pens, so you are saying:

I have twice as many pens as your pens have.

Pens don't have pens, so you see, that's very strange!