Well, the historical setting of the movie is true. The fact that the Nazis were in power during WWII and there were resistance efforts is real. But the movie's main plot, including the Basterds' extreme revenge - seeking missions and the way the story unfolds in the movie, is mostly fictional. So, I'd say less than half of it has any connection to true stories. It's more of Tarantino using history as a springboard for his own narrative.
A relatively small fraction. The movie takes the broad strokes of WWII, such as the occupation of France and the general brutality of the Nazis. But the specific story of the Inglourious Basterds, their actions like scalping Nazis and the way they infiltrate the movie premiere, are all made - up. So perhaps around a quarter of the movie can be said to be loosely based on the true idea of anti - Nazi resistance during the war.