It's whatever program you used to make the STL. Most CAD programs allow you to specify the accuracy when converting to STL. If you specify something reasonable for Reprap, i.e. 0.1mm, it is surprising how few sides you need to keep within that accuracy.
For aesthetically nice looking circles you need more sides because your eye is sensitive to the light changes you get from the different angles of adjacent facets. Some CAD programs also let you specify an angle accuracy and you need something below 5 degrees to get smooth looking circles. Ironically that produces vertices that deviate far less than 0.1mm from the true circle, which is far more accurate than the machine is.
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