German Empire: 2,527,000
Remember this is only part of the total flow, since many emigrants went to other parts of the New World.
Some of these people came back, but probably only one out of ten; the rest never returned. Not only are these numbers enormous, they came primarily from certain districts. Much of the total for the German Empire, Austria Hungary, and Russia was made up by 3,800,000 Poles, and most Italian emigrants came from the south. The impact on those districts was profound:
Emigration to the New World sometimes had a dramatic effect on Europe itself and bizarre political hopes were fostered by the links that grew out of it. During the 1930s officials in the Polish foreign ministry fantasized about the prospect that emigrants from their country might found a colony in Latin America, while in 1945 some Sicilians proposed that their island might become a part of the United States. More seriously, central European nationalism was cultivated. During the First World War, a legion of Polish volunteers was raised among emigrants in the United States, and the influence of emigrants on eastern European politics was to persist for the whole of the twentieth century. In 1990, Franja Tudjman's campaign to become president of Croatia was said to have raised around $5 million from emigre supporters. In the same year, Stanislaw Tyminski, who had made his fortune in Canada, returned to Poland to run for president.– Richard Vinen, A History in Fragments: Europe in the Twentieth Century (2000), pp. 17, 20.
Emigration had less obvious effects on parts of Europe. It increased literacy, because families needed to keep in contact by letter. It also created imbalances of gender and age as young men left: between 1905 and 1916, 4.86 million Italian men emigrated, but only 1.14 million women accompanied them; in early twentieth-century Calabria, there were three young women for every two young men. Sexual imbalance may have produced a self-perpetuating cycle. Carlo Levi suggested that extra-marital sexual relations in parts of the Italian south were common because there were not enough men to provide all women with husbands. Illegitimate children in turn were particular prone to emigrate – in one well-studied village, three quarters of them did so.
Migration increased prosperity in home countries as money was sent back or as emigrants returned to buy cherished plots of land. The economic impact of emigration was particularly great in Italy, where links between emigrants and their places of origin remained close – it was said that Italy gained $100 million from emigrants who returned between 1897 and 1902. An enquiry of 1931 showed showed that 2 million hectares of land were bought by Italians returning from America. . . .
Emigration was often linked to political conservatism. Generally the areas that sent emigrants abroad during the early twentieth century remained on the political right for the rest of the century.
According to Vinen's numbers, Italy lost 6 million emigrants in eleven years, from a population of around 32 million. According to wikipedia the total outflow of people from Europe over the period 1810 to 1932 was at least 60 million, and most of that happened after 1880.