In the modern era, Vic Toweel scored 14 knockdowns of Danny O'Sullivan in defense of his bantamweight title in 1950. Freddie Steele knocked down Vince Dundee 11 times back in 1935. Here's that fight: [yt]NUL-6M9wKwA[/YT]
This is the first one that jumped to mind. Before around 1960 is really different though, because before then the fighters didn't need to go to the neutral corner. They could just hover over the guy who was knocked down and keep knocking him down until the round ended.