usually, 100 strands a day, but hair falling depends on hair growth phase, there are phase when hair grow and when hair falls, for example: "The Telogen Phase - This is also known as the resting phase when the hair begins to fall out and about 10-15% of the hair on your head is in this phase at any given time. At this point the follicles are weak and thin and can be pulled out very easily. After 3 months at the most hair in the telogen stage will fall out on its own, making way for new healthy growth that is in the all important anagaen phase." full article: http://hair101.org/hair-growth-cycle/
The exact number of hair shed varies from person to person. The normal hair fall off in a day is about more than 100 strands. Excessive hair loss may indicate an underlying diseases such as fungal infection, lupus or diabetes.