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I was expecting a complicated equation
There is a complicated equation underlying it, and (hole count)/9 fails if the hub's hole PCD is too large a fraction of the rim's ERD, but for common hubs and 26-28" rims it works. Things like small wheels (BMX, Brompton etc.) or big hubs (Rohloff and upwards) should be calculated rigorously if you want to use the maximum number of crossings.
Maximum 2. In general, it's the integer part of hole count/9