Sorry, I am a n00b and don't know anything about music theory. Are typical lead synthesizer notes in trance played as single notes or as chords?
It depends! "Lead synth" typically connotes a single note sound ("mono", short for monophonic) but you'll also find lead synths sounds that are "poly", short for polyphonic, depending on the (sub)genre, the artist's style, what the artist had for breakfast that morning, etc.
The most common lead synth in trance is probably a mono synth constructed from two (or more) detuned saw waves with delay and reverb, but there are plenty of examples of poly sounds used as leads as well.