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Can someone say how rare of an alignment this would be? Once a year? Once every few months?

That's an interesting problem. The mountain peak/cathedral line of sight is always there. They fortuitously aligned in a direction at which the moon may set. Using google maps, I could discern that the bearing from the Basilica to the mountain peak was about 229.027 degrees. Scaling off google maps and the photo, it was taken about 10 kilometers northwest of the Basilica, which again, from google maps, happens to be the location of a park, with hilly hiking trails, some with a good panoramic view of Turin. Assume that there is a place to stand and obtain that view. All of these are givens, conditions that apply 100% of the time

Judging from the aesthetics of the picture, you want the moon centered to about 2% of its width, or approximately plus or minus 0.01 degrees. From NASA's JPL horizons, I learn that the azimuth of the setting moon at Turin varies between 229.008003 and 313.253893 during 2023 with almost a completely sinusoidal variation over the 29.53 day lunar cycle. On December 15th, it "set", reached the sea level horizon at 19:21 local time, at bearing of 234.186. It was at 229.027, and 9.65 degrees above sea level elevation at the time that the picture was taken. If we assume that the variation in azimuth at 9.65 degrees elevation is more or less the same as the variation at sea level (a reasonable approximation) then the fraction of the time that is sets at the desired azimuth is 0.000412977. This is probability that the moon will be withing 0.01 degrees (1/50 of the moon's angular width) at moonset on any particular day, or roughly once every 2400 days or once every six and a half years. This could occur at any point in the lunar cycle, full moon, crescent (as in this case), or new moon, with the sun up and sun down. If you only count days when the sun is down, more like once every 13 years. If you also provide constraints for weather, and aesthetics (a full or gibbous moon would look washed out), this is more like a once in a lifetime shot!

If you only count days when the sun is down, more like once every 13 years.

According to my calculations, this configuration (phase of the Moon, position in the sky of the Moon relative to the cathedral, as viewed from that particular spot on the Earth's surface where the photographer stood) would recur every 6,940 days (i.e. approx. every 19 years).

Regards,