Passage: These are rare objects; it is estimated that there are no more than 20,000 class O stars in the entire Milky Way, around one in 10,000,000 of all stars.  Class O main sequence stars are between 15 and 90 M☉ and have surface temperatures between 30,000 and 50,000 K.  Their bolometric luminosities are between 30,000 and 1,000,000 L☉.  Their radii are more modest at around 10 R☉.  Surface gravities are around 10,000 times that of the Earth, relatively low for a main sequence star.  Absolute magnitudes range from about −4, 3,400 times brighter than the sun, to about −5.8, 18,000 times brighter than the sun.Class O stars are very young, no more than a few million years old, and in our galaxy they all have high metallicities, around twice that of the sun.  O-type main sequence stars in the Large Magellanic Cloud, with lower metallicity, have noticeably higher temperatures, with the most obvious cause being lower mass loss rates.  The most luminous class O stars have mass loss rates of more than a millionth M☉ each year, although the least luminous lose far less.  Their stellar winds have a terminal velocity around 2,000 km/s.

Question: How many class O stars is it estimated that there are?
20,000