Reliable quantitative data on WW2 is surprisingly hard to track down, since (I suspect) few governments would have been keen to publicize mid-air collision as a frequent event. However, collisions clearly occurred: there are reports of German pilots dying in collisions in WWII. It's not clear from the source I have found, though, whether those were mid-dogfight collisions or manoeuvring errors (https://books.google.ca/books?id=RSMeCgAAQBAJ&pg=PA151).

From what I have read, although I don't have the precise source to hand, mid-air collisions outside of dogfights were more common among densely packed bomber groups, especially flying at night or in poor weather conditions (the Soviet author Konstantin Simonov mentions the difficulties of this kind of flying in his wartime article "U-2").

More anecdotally, in my very extensive reading around pilots' and soldiers' bibliographies on all sides during the war, I have very rarely come across collision as a major issue during dogfights. If anyone has sources that suggest otherwise, though, I''d gladly accept I'm wrong.