The writing sample is to show a sample of your research. Just because you mention multiple papers or projects in your statement of purpose does not mean you need to include them all in your writing sample. Pick what you think is the single best paper and include that. (As jonescb suggests, if the application guidelines give a page limit, and you have several short samples, you could consider including more than one, but only if their total length is less than the stated page limit.)
Do not (as you suggest in your comment) "exploit" the writing sample to stuff in as much of your research as possible. If you have published, that will show up in your CV. If your samples are not published but were, for instance, written as class papers, that will show on your transcript in the list of classes you've taken and how well you did in them. The writing sample is not supposed to be "proof" that you've done everything you talked about your SOP; it's just a sample of one thing you've done.
Even if the ocmmittee does look at your overlong writing sample, it will probably be perceived negatively. A person who tries to stuff the sample may be perceived as unable to focus on a single topic, or as trying to show off how much they've done. At the least, you will probably be perceived as someone who did not pay attention to the application directions (which, in my experience, usually use the world "sample" in the singular and give a rough page range), which never helps.