10 spreads is long enough for a full portfolio.  If 10 spreads is your "sample", how long is the full version? Seriously, even in an interview employers don't expect to go through 30 projects.  When I interview anyone with a portfolio of more than 8 pages or so, I usually say "can you tell me about your favorite project?" and try to limit most of our portfolio review time to that.  Send the 10-spread version, or an even shorter teaser (one or two projects illustrated on 1 to 4 pages is appropriate).

For some reason most applicants put their favorite project last in their portfolios.  Don't do that, at least not in a version that you mail, because if the reviewer gets bored on the 2nd project they'll set your portfolio aside and never see your favorite Project #10.  

I'm launching into old-timer mode now, but: I think the ability to create portfolios digitally has made it too easy to avoid self-critique in culling down the thing to only your best work.  Unless you have a lot of built work of many different project types, all of which you know the target firm also works on, you don't need more than 10 projects.  Back in my day, creating a portfolio meant x-actoing original photos, and scaling drawings on a xerox machine and cutting and spray mounting them, and doing this individually for each copy of our portfolio.  I wouldn't want to go back to that, but I do believe the limitations that this created in having the time and money to create a limited number of spreads helped us to self-edit to only the strongest work.