What you bring to your interview should be a comprehensive overview of everything you’ve done. There is no set number of pages, as no student has the same amount of work.
With that said, don’t show stuff thats crap. It’s ok to show a project that may have a really really strong aspect (IE really strong foam prototypes) but might be weak in another regard (bad sketches). In that kind of situation you’d be best served by highlighting the strong and leaving out the weak aspects.
For sketches - it’s useful to show compiled edited sketch pages in your portfolio, you want the sketches in your portfolio to show where you started, how you progressed, and where you ended up – and do this all visually. For that you’ll need to clean them up and organize them…pay attention to size and weight, so that the big ideas stand out more then the ideas you just sketched for the sake of pleasing the teacher.
When I intereviewed I ALSO brought a book of entirely unedited sketches, just a bunch of 11x17" sheets organized by project. This binder also included stuff that had no relevance (car drawings, figure drawings, photos) to my actual body of work, but the main point was to show the employer (and the people I was presenting to) that I could draw.
Just as a laundry list of what I brought to my interview:
Book of sketches/small projects/photos/press releases for competitions I had entered - 11x17" ~50 pages
Digital presentation ~45 slides in powerpoint - showing process and key projects
Research book - short book focused specifically on user research
Project book - 15 page competition entry booklet for what I thought was my strongest project (that wasn’t all covered in the presentation)
Business Plan - a business plan I had written for a bike based business venture I had done in school
Digital copy of my entire website (assuming I’d have no internet access)
I think it worked to my advantage (since I got the job) because in addition to showing my work I had a whole bag of goodies I could whip out. For example when I interviewed directly with the head researcher I was able to focus on my research book. During the presentation I had to the entire group, I was able to hand out all of the materials so everyone had something to flip through while I was talking (kept the people who might have been uninterested in the presentation interested in something related to my work).
Obviously you may not have all those things available, but having a collection of different materials you’ve done is a valuable resource.