These are estimates, but here's a rough summary:
- 12 million Germans from more or less every Eastern and Central European country.
- 2.1 million Poles from the Soviet Union.
- 650,000 Ukrainians from Poland.
- 250,000 Italians from Yugoslavia.
- 145,000 Hungarians from Czechoslovakia.
- 80,000 Hungarians from Yugoslavia and Serbs and Croats from Hungary.
- 70,000 Slovaks from Hungary.
So according to that estimate, about 15.3 million in total, though I expect some will differ slightly.
Source: Philipp Ther, The Dark Side of Nation-States.