Three.
A German physicist, Gabriel Fahrenheit, developed the Fahrenheit Scale in 1714. On this scale, 0 degrees equals the lowest temperature he could measure in his lab. He assigned 32 degrees to the freezing point of water. The boiling point (also called the steam point, because it's the point at which water begins to turn to steam) is recorded at 212 degrees.
Anders Celsius, a Swedish astronomer, invented the Celsius Scale in 1742. On this scale, 0 degrees equals the freezing point and 100 degrees equals the boiling point.
A third scale used for scientific purposes is the Kelvin Scale, in which the freezing point is 273 degrees and the boiling point is 373. All those other degrees below 273 are used for making very exact scientific measurements, with 0 degrees equaling absolute zero, the point at which molecules are assumed to cease all motion.
Fahrenheit measurements are most common in English-speaking countries, although outside the U.S., its use is declining. Celsius is used almost everywhere else and is considered easier to use, because it's two most significant measures are the easy-to-remember "0" and "100".
---David Murphy
