Very interesting improvisation by that elder, Bonnie_Clyde. Depends on how you want to count.
From the book, A World Undone: The Story of The Gret War 1914-1918, by G.J. Meyer, we have a handy little timeline which I quote in part below, along with a running tally of belligerents.
July 28: Austria-Hungary declares war on Serbia (2)
Aug. 1:Germany mobilizes, declares state of war with Russia (4)
Aug. 2:German troops enter Luxembourg (5, but no actual declaration of war)
Aug. 3:Germany declares war on France (6)
Aug. 4:Germany declares war on Belgium (7)
Aug. 12:Britain declares war on Austria-Hungary (8)
Oct. 29:Turkey enters war on side of Austria-Hungary and Germany (9)
Of course, Monenegro and Japan declared war during this time period. Making the number 11, if you count Luxembourg.
So, maybe drop Luxembourg, since there was no official declaration of war. So, you get 10 again! On the other hand, The Ottoman Empire actually declared war in November (or had it declared on them), not in October. So, now we are back down to 9.
Well, 9, 10, or 11 are all very close to 10. So prophetic completeness is confirmed!