This is rather a complex matter!

"Number of notes
One way of classifying chords is according to the number of distinct pitch classes used in their construction, a pitch class being identified by a degree of the chromatic scale (that is, a certain musical note, such as A, B, C, D, etc.) without regard to which octave it occurs in. Chords using three pitch classes are called trichords. Chords using four notes are known as tetrachords. Those using five are called pentachords, and those using six are hexachords."

http://tinyurl.com/2cdsuh