Yes, my friend, it is difficult to find any information on
current or historical attendance at Keirin races in Japan.
I have read in recent articles that the attendance at the
Sydney Cricket Grounds in March of 1903 to watch the
Sydney Thousand (that I referenced above) was 54,000.
In another article the attendance was given as 35,000.
Neither article gave a source for their number, although
the higher number was in a piece on the Cycling NSW
web-site, so they may have a reliable historical source.
The only historical reference I found stated "scores of
thousands" which may indicate at least 40,000 I guess,
although making any assumptions from a newspaper
story that is 110 years old is risky when one cannot
be sure of the usage and meanings of certain words
and phrases during that time period.
The SCG cycling track which was built in 1896 was
removed in 1920 according to Wikipedia, however no
mention is made of the size of the cycling crowds.