Trial by jury is traditional in this country, but it's not protected by the Constitution. Trial by jury is guaranteed in criminal cases in the Sixth Amendment and in most civil cases in the Seventh Amendment. Many states have set the number of jurors at fewer than 12, at least for certain types of cases. The U.S. Supreme Court has explicitly permitted juries of as few as six members. It has not done so with juries of under six members.
Juries can only render a verdict if the vote is unanimous.
Juries can certainly render verdicts if unanimous. It refused, however, to uphold a 5-1 verdict. Juries have to be based on a representative cross-section of the community. The pool from which they are chosen should be community wide, and no person should be excluded on the basis of race, gender, or other impermissible purpose.