Fahrenheit based his scale on what he thought to be absolute zero (i.e. the coldest temperature he could produce in his lab with the tools of his time) and his body temperature, which he set to 12, because 12 was a convenient number and used in a lot of scales in his pre-metric time. He did realize though that this scale was impractical, and halved his degrees until they deemed sensible to him, resulting in the final degrees to be ⅛ of the first draft. 8 * 12 = 96, hence 96° F was his second fixed point.

Which is just senseless, as we know today, as the temperature of the human body fluctuates over time. If we took the original definition seriously, everybody would have their own Fahrenheit scale that would differ over time.

Fahrenheit is not based on body temperature, it is based on the temperature of a mixture of ice and salt and the body temperature of a certain individual, both in 1714. Who was, by the way, suffering from hypothermia.