A lot. Just looking it up on the internet: in 2002 one man in the US had donated 213 units over 50 years time; and in 2010 in Australia it is reported that one man had donated 231 units of blood by age 75. One person would typically donate one unit of blood at each time of donation.
Great question. One pint every 6-8 weeks after age 18 until age 70? 6xper year x 52 years=312 pints/8=44gal there are gallon clubs so someone is measuring. American red cross might really know.