There is a source for 18000 members in 2017 in the Wikipedia article [0], see footnote 11. The "Bundeszentrale für Politische Bildung" [1] is a reputable source.

[0] https://en.wikipedia.org/wiki/Grey_Wolves_(organization)#Ger...

[1] https://en.wikipedia.org/wiki/Federal_Agency_for_Civic_Educa...