It's not clear how Fahrenheit arrived at the scale he did. The most likely reason (one he described himself) is that 0° is the temperature at which a mixture of ice, water and ammonium chloride (a so–called frigorific mixture) stabilises, and 96° was his own body temperature. He seems to have chosen 96°, rather than 100°, as it meant that the intervening range could be divided into eight regions of 12 degrees. The reason why this is more convenient than ten ranges of ten is presumably because it's more easily divided into further sections.
© Haydn Thompson 2017