1. It's waaay more important what your letter says than from whom it's addressed.
2. If you're wondering whether or not to ask someone for a recommendation, don't. Only ask people you KNOW will write glorious things about you.
3. Submit the recommended number of letters to the schools to which you apply - many require fewer than they recommend, but will accept even more than they recommend. Submit the number of letters the school recommends.
Hit me with questions.