I submitted a 6"x8" portfolio with 48 total pages. It was double sided.

I had four featured projects and a few extra artwork, photography, and smaller projects.

Overall there were 6 sections total, each with a spread dedicated for the title page. I also had a spread for the table of contents.

I believe that 45 is a bit thick and harder to work with though because you have to design so many spreads. The reason mine ended up with so many pages was because I like to use a lot of white space. Typically with the portfolios I've seen, they are much too cluttered and I chose to stay away from that.

Depends on what schools you are applying to. Some have no stipulations, others are very strict on what the format can be (although some would argue in favor of breaking those stipulations). Personally, I'd opt for something simple that can be easily added to or abridged depending on what its needed for. After revising my portfolio an uncounted number of times for schools, job interviews, etc., I think flexibility is the most important thing. In other words, don't base your portfolio on a rigid format.

Also, do a search of portfolio discussions. This topic tends to come up a lot...