For a long time Magneto didn’t have a name. He had Magnus as an alias and that’s it.
The entire thing about Erik Lehnsherr came about because at one point Marvel didn’t want Magneto to be Jewish, so they gave him a Gypsy name which is what Erik Lehnsherr is. They even went as far as to change the numbers on his forearm so it corresponds with a Gypsy concentration camp and edited his history.
Claremont felt it was important for him to be Jewish so he threw away Erik as an alias a few years back. Whenever Claremont got his hands on Magneto he would toss the Gypsy thing out. He feels his backstory of being a concentration camp survivor doesn’t really resonate unless Magneto is Jewish. I agree with him there.
Some writers like Bendis, Hudlin, etc. were not aware that Erik was an alias so they used it as his real name a few times in the narration which you shouldn’t do, but it wasn’t his real name. Current X-Men writers like Carey and Fraction are aware of the name thing so these problems don’t show up in X books.
When Magneto Testament came out it confirmed that Magneto was 100% Jewish and his birth name is Max Eisenhardt. A relative of his in that story had the first name Erik which is probably why Magneto chose Erik as an alias.
No one within Marvel comics (Cyclops, Professor X, etc.) is aware of his birth name, they assume that it is Erik. Obviously the all knowing types like The Watcher probably know his birth name and stuff but there haven’t been any instances where anyone has called him Max in the current era.
The entire thing is strange because Marvel already has a Gypsy villain, Doctor Doom… I guess no one was lobbying for Gypsies at Marvel…
All of this only applies to 616, the mainstream Marvel comics. His name is Erik in cartoons, movies and everywhere else. His ethnicity doesn’t seem to come up in those things anyway.
In Marvel vs. Capcom 3 they do list his name as Max Eisenhardt for his real name under his bio though.
Marvel’s been oddly territorial with Magneto for some reason. They changed Morrison’s X-Men story to make it so Xorn wasn’t Magneto, because they had a problem with him being a mass murderer. You’d think Magneto was Spider-Man with how concerned Marvel is to how he appears. Go fig.