In 2002 10% of children in the UK did some sort of martial arts training http://www.stats4schools.gov.uk/images/SP_Young-People-and-Spo-131650_tcm86-13172.pdf You could try adding up all the big organizations figures but I still doubt you'd get anywhere near the total amount.
i had heard that martial arts was the 3rd biggest "sporting" past-time, behind fishing and football...