Actually in Europe there were likely as many Caucasians slaved from the coast lands around that continent from the 16th through 18th century to North Africa around the time when slaves were imported in the US; it just is one of those subjects never spoken of. The numbers were high because the average life span was 3 years due to the brutality of the conditions and no attempt was made to keep them alive and there are scarce records kept in those places. When reading Joan Peter’s From Time Immemorial most of the documents regarding Jewish persecution and slaughter came from Western diplomatic activity in those areas in the 19th century. Keep in mind 95 percent of slaves from Africa went to S. America and the Caribbean.
Robert C. Davis documents 1.25 million to 1.5 million Europeans slaved mostly from Italy during 1500 to 1800.
Christian Slaves, Muslim Masters: White Slavery in the Mediterranean, the Barbary Coast and Italy, 1500-1800 (Early Modern History