Has an Euros game ever ended 12-1?
Exactly once. On December 21, 1983, Spain beat Malta 12-1, and no Euros game before or since has ended that way. When it happened it was a scorigami, the first 12-1 in league history.
That makes 12-1 the 28th most common final score out of 34 that have ever occurred in Euros. One point away, 13-1 has happened never and 12-2 has happened never.