They forced all government employees to become members. At a certain time if you wanted to advance anywhere in society you had to be a member. So the numbers are inflated anyway.

Also, fyi, it seems a lot of Nazi party members were jewish. Supposedly the best documentation is a book called 'Bevor Hitler Kam', but I haven't had time to obtain it. There is a machine translated copy available on the net though.

There were six million nazi's, in germany.