How many Aikido members are there?

Hello to all of you,

I am a student from Germany. I have to do a presentation in my university, and can choose the subject. And guess what: Aikido!

So, to do a good job, I plan to speak about the history of Aikido. I want to show how the numbers of active Aikidoka changed during the years, but can't find any sources in the net! Has anybody an idea where I can get this information, form internet or books?

Many thanks in advance!
Timo