Should I use a different circle for each part that I record or should I record multiple parts within one circle?
Both methods work.
Each "circle" is a track, and if you're familiar with loopers like the Ditto from TC Electronics,
each track = one Ditto.
You can record and overdub in a single track.
With that said: I recommend you make use of the multi track capabilities of Loopy and use one track per part.
(otherwise, why bother with loopy, Just take any other simpler app or a looper pedal)