I rarely rant, but here we go...
Not to be a bastard naysayer, but the advice in Mashable's "How To: Score A Design Job" is so frustrating that if I facepalmed, I'd likely push my face through the back of my head. These types of articles have been written a thousand times. They always include the same advice and always provide zero context (ie. what type of design job/business is this advice suited for?). Nothing is more annoying than "rules" for up-and-coming designers on how many pieces to put in your portfolio, how many pages your resume should be, or what the content of your cover letter should be.
THERE IS NO RIGHT ANSWER.
I used to do portfolio review for graduating seniors at Columbia College in Chicago and almost every kid came in with the exact same set of questions. I'd tell them all the same thing: "Do you what you feel is right and treat each situation on an individual basis." Everyone else's opinion about what's right is based upon two things: (1) what they'd expect if they were hiring, or (2) what worked for them when they were hired – and yet they're always represented somehow as globally applicable, chiseled-in-stone fact.
This is my advice: Take all advice (including mine) with a huge grain of salt – what works for one person usually will not work for others. And a few pointers...
Put as much work in your book (online or off) as you feel best represents your skills, whether it's 10 or 100 pieces. Present what you're proud of. You get hired based upon the quality of your work, not a judgment of the quantity.
Put whatever you feel best represent your career path on your resume. Mine is 2 pages long. People who think it needs to fit on one page can kiss my ass. However, beyond some simple layout and type, don't design your resume. Seriously. Its purpose is to convey information, not your design skills. That's what your portfolio is for.
Cover letters are stupid. You're not being hired for your writing skills. Sometimes they're required, so in those cases, anything beyond "Hey, check out my work and let me know if you think there could be a fit" is a waste of time. There is nothing else you can say that will get you an interview if your work isn't worthy of one. Save your spiel for in-person when you'll be judged on whether you're a good cultural fit for a team. If a cover letter is required, think about the type of place that would require it, and consider whether you want to work there. Nothing says oil and water more than designers and formality.
Lastly, do whatever you want. Nothing is more miserable and uncomfortable than failing when you were only doing what someone else told you to do. The whole point of being a designer is to solve problems... you should be able to figure this stuff out on your own. And please, please, please when you do figure it out for yourself, don't fool yourself into thinking your solution is the solution.
Some call me a tattooed metal head with an eye for design and a nose for tomfoolery. I call myself a tireless design enthusiast, a champion of users, a designer of products and experiences, an advisor to startups, an avid consumer of food, movies,and tee shirts. A husband, and a maker of things.
You can just call me Jeffrey.