|MadSci Network: Anatomy|
the short answer to your question is, between zero and 150,000.
With these numbers, we are counting only the so-called terminal hair growing on people's head. We do not count eyebrows, eyelashes,, nasal and aural hair, beards and pubic hair (these are made of terminal hair as well); neither do we count the short, fluffy unpigmented hair strands that grow everywhere on one's body except the lips, ears, palms, soles, navels, scars and parts of the external genitalia (that hair is called vellus hair).
Giving the lower limit as zero is significant, as there are a number of conditions which can cause one to lose all hair on one's head. One of those conditions is old age in some men; but many diseases can cause loss of hair to varying extent.
Barring these conditions and assuming healthy, young or middle-aged subjects with healthy head hair, the number of hair strands on one' head is about 150,000 at most. The numbers vary mostly by hair colour, although genetic influences also play a role. Blonde people have about 150,000, black-haired people 110,000, brunettes 100,000, while redheads have to make do with about 75,000. Generally, the thicker the individual hair strand, the lower the number of all hair strands is.
We lose about 60-100 hair strands a day. In a healthy individual, you find about 200 hairs per square centimetre; growing at about a third of a millimetre a day. A single hair follicle, if not torn out, will go on to produce hair strand for about 6 - 8 years before it shuts itself down.
For further reading, any good dermatology textbook will get you going. I plucked my numbers from my old 90's edition of a German textbook. Also, the German Wikipedia page (as opposed to the English version) explicitly states these numbers, if you need a quick reference.
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