So let's think:
20 years race at Nordschleife (1950-1969), Hockenheim in 1970, 6 more at Nordschleife (1971-1976) There also was a GP at Avus in late 50's-early 60's so -1 for Nord.. Until now we have 25 N - 1 H
Then 20 years at Hochenheim (1977-2006) and 3 more later (2008, 2010, 2012). 26 N - 24 H
Also, I remember the onboard camera live debut (how odd
) in 1985 with Hesnault, so a GP or two must have been held back in the Mid 80's. Random.org says 1, so I will go for one GP in the 80's there. Also we had the 2009, 2011, and 2013 races.
So Nurburgring 29 (25 Nord. - 4 Nurb.) - Hockenheim 24 = 6
I must have put some GPs while they didn't happen.