Is a chord a set of three pitches played at the same time or a chord is a set of any number of pitches played at the same time?

A chord can technically be two pitches played at the same time - an example would be C5, which is just C and G. But most of the time we talk about chords, we're talking about triads (3-pitch chords), like C major (C, E and G). 

Then, we can start adding extensions like 7ths, 9ths, 11ths, 13ths, etc. We can also alter the chord (for example, sus4 or add2). 

But usually we start with simple triads: major, minor, augmented or diminished!

Okay,thank you 

Happy to help!