The number of voters (from the registered grouping) is always an interesting topic.
The history:(note, I'm using West German numbers, and Germany-united)
1953: first election after WW II, 83-percent turn-out
1957: 87-percent turn-out
1961: 87-percent turn-out
1965: 86-percent turn-out
1969: 86-percent turn-out
1972: 92-percent turn-out
1976: 90-percent turn-out
1980: 88-percent turn-out
1983: 89-percent turn-out
1987: 84-percent turn-out
1990: 78-percent turn-out
1994: 79-percent turn-out
1998: 82-percent turn-out
2002: 79-percent turn-out
2005: 77-percent turn-out
2009: 71-percent turn-out
2013; 71.5-percent turn-out
I sat and watched some political chat forum on German public TV back in mid-June, and one of the older guys on the forum commented that he'd never seen such a low-intensity campaign.....such as it is right now in Germany. There's virtually no hyped up topics....no public enthusiasm....nothing which would typically get people to show up and vote.
There are roughly 62-million Germans registered to vote.....meaning that twenty million residents in Germany....either are foreign citizens with no vote, or just Germans who have zero interest in politics (yes, there are some).
If you go back to the numbers of the 1950s and 1960s....it's an amazing number....over 90-percent of folks showing up and voting.
Since the Wall came down? That's the interesting thing....you can say that roughly 10-to-15 percent of the usual crowd of voters....aren't showing up for the past six elections.
As for the election in roughly four weeks? I'd take an educated guess that it will be the lowest vote situation since WW II...somewhere between 66-and-70 percent.