Many years ago, when dinosaurs ruled the earth, music
was recorded on vinyl
discs. (Still is, in fact, but now it's a specialty product.)
Each disc generally had two grooves -- one on either side -- each of which spirals inwardly in such a narrow pattern that it looked almost like concentric circles.
As a result, it became a piece of second-class wit to ask a mark how many grooves were on a record. If you thought it through, you'd realize that there were two.
However, some engineers have occasionally decided to push the envelope. A few albums have two interlocking grooves on one side, and unless you pay attention, it's a matter of chance as to which set of songs is played.
In the 1960s, there was a novelty record that would let you play roulette at home. It had 38 grooves on a side. To use it, you let the LP spin on the turntable for a little while, then brought down the tone arm to hear which number had been randomly selected.
Next time some wiseguy asks you this question, you can say: "Two... except when there are more."