According to the FIFA website, (http://www.fifa.com) more than six million people are active in 27,000 soccer clubs throughout Germany. The World Bank puts Germany's population around 82 million people as of 2008. (http://data.worldbank.org/). Using this data, roughly 7.3 percent of Germans play soccer. Though I'd assume there are more players who aren't active in soccer clubs.
Hope this helps! :)